It will enter tapped. Heliod will count its own mana symbol to the devotion.
The relevant Gatherer-ruling of Heliod:
9/15/2013: If a God enters the battlefield, your devotion to its color (including the mana symbols in the mana cost of the God itself) will determine if a creature entered the battlefield or not, for abilities that trigger whenever a creature enters the battlefield.
Edit: I can't read. See following answers for correct ruling. Heliod enters untapped.
It will enter tapped. Heliod will count its own mana symbol to the devotion.
The relevant Gatherer-ruling of Heliod:
9/15/2013: If a God enters the battlefield, your devotion to its color (including the mana symbols in the mana cost of the God itself) will determine if a creature entered the battlefield or not, for abilities that trigger whenever a creature enters the battlefield.
Unfortunately, this is incorrect. That ruling has to do with triggers and Thalia doesn't have a trigger; she has a replacement effect. Since replacement effects have to take effect before the event actually happens, Heliod will not count his own mana symbols for Devotion. In the original scenario, Heliod will not enter tapped (since Devotion is only 4 before he enters) and will then be a creature on the battlefield that will trigger any abilities based on a creature entering the battlefield.
Here is the relevant rule:
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.
I bolded the important section above. This line can be misleading, but in this case it means that you determine if Heliod is a God with the present board state. If not, Thalia's replacement effect won't apply.
That ruling doesn't apply (post #2), since the scenario doesn't involve any triggers. Triggers usually look at the game state right after the trigger event happens, and ETB triggers aren't exceptions. So after the god enters, the game looking for ETB triggers will see a creature that entered the battlefield. Here we have a replacement effect, though, which are applied before the event happens.
I know that a similar question has been asked a lot after Theros came out, but I couldn't find any thread where it was answered. But there usually was a lot of debate about wether or not the game takes into consideration the god's mana cost as it would exist on the battlefield. I think (!), that the consens was, no it doesn't, and that even though the god will enter as a creature, it will enter untapped.
(This is just a correction on the above post as to why the argument isn't valid. I'm not certain as to the correct answer, hence why I didn't answer in the first place.)
It will enter tapped. Heliod will count its own mana symbol to the devotion.
The relevant Gatherer-ruling of Heliod:
9/15/2013: If a God enters the battlefield, your devotion to its color (including the mana symbols in the mana cost of the God itself) will determine if a creature entered the battlefield or not, for abilities that trigger whenever a creature enters the battlefield.
Unfortunately, this is incorrect. That ruling has to do with triggers and Thalia doesn't have a trigger; she has a replacement effect. Since replacement effects have to take effect before the event actually happens, Heliod will not count his own mana symbols for Devotion. In the original scenario, Heliod will not enter tapped (since Devotion is only 4 before he enters) and will then be a creature on the battlefield that will trigger any abilities based on a creature entering the battlefield.
This goes into more detail but, again, because a replacement effect has to replace an event before it happens, Heliod will not be on the battlefield to count its own mana symbols. I also understand that this rule is 3 years old (since the article came out when Theros came out) but it hasn't changed since then.
It will enter tapped. Heliod will count its own mana symbol to the devotion.
The relevant Gatherer-ruling of Heliod:
9/15/2013: If a God enters the battlefield, your devotion to its color (including the mana symbols in the mana cost of the God itself) will determine if a creature entered the battlefield or not, for abilities that trigger whenever a creature enters the battlefield.
Unfortunately, this is incorrect. That ruling has to do with triggers and Thalia doesn't have a trigger; she has a replacement effect. Since replacement effects have to take effect before the event actually happens, Heliod will not count his own mana symbols for Devotion. In the original scenario, Heliod will not enter tapped (since Devotion is only 4 before he enters) and will then be a creature on the battlefield that will trigger any abilities based on a creature entering the battlefield.
Here is the relevant rule:
614.12. Some replacement effects modify how a permanent enters the battlefield. (See rules 614.1c–d.) Such effects may come from the permanent itself if they affect only that permanent (as opposed to a general subset of permanents that includes it). They may also come from other sources. To determine which replacement effects apply and how they apply check the characteristics of the permanent as it would exist on the battlefield, taking into account replacement effects that have already modified how it enters the battlefield (see 616.1), continuous effects generated by the resolution of spells or abilities that changed the permanent’s characteristics on the stack (see rule 400.7a), and continuous effects from the permanent’s own static abilities, but ignoring continuous effects from any other source that would affect it.
I bolded the important section above. This line can be misleading, but in this case it means that you determine if Heliod is a God with the present board state. If not, Thalia's replacement effect won't apply.
You are correct. I read a comma, when there isn't one. I found a judge blog article, answering the same way you do, with an explanation why. So ignore my first answer.
It will enter tapped. Heliod will count its own mana symbol to the devotion.
The relevant Gatherer-ruling of Heliod:
Edit: I can't read. See following answers for correct ruling. Heliod enters untapped.
Or are you interested in a Fiora flavor cube? Conspire and win!
Level 2 Judge
Here is the relevant rule:
I bolded the important section above. This line can be misleading, but in this case it means that you determine if Heliod is a God with the present board state. If not, Thalia's replacement effect won't apply.
I know that a similar question has been asked a lot after Theros came out, but I couldn't find any thread where it was answered. But there usually was a lot of debate about wether or not the game takes into consideration the god's mana cost as it would exist on the battlefield. I think (!), that the consens was, no it doesn't, and that even though the god will enter as a creature, it will enter untapped.
(This is just a correction on the above post as to why the argument isn't valid. I'm not certain as to the correct answer, hence why I didn't answer in the first place.)
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
http://blogs.magicjudges.org/rulestips/2013/10/gods-4-devotion-and-entering-the-battlefield/
This goes into more detail but, again, because a replacement effect has to replace an event before it happens, Heliod will not be on the battlefield to count its own mana symbols. I also understand that this rule is 3 years old (since the article came out when Theros came out) but it hasn't changed since then.
http://www.mtgsalvation.com/forums/magic-fundamentals/magic-rulings/magic-rulings-archives/513623-gods-dudes-tapped
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
You are correct. I read a comma, when there isn't one. I found a judge blog article, answering the same way you do, with an explanation why. So ignore my first answer.
Here the link to the judge blog Judge blog article.
Or are you interested in a Fiora flavor cube? Conspire and win!
Level 2 Judge