Enchanted creature can't attack or block, and its activated abilities can't be activated.
At the beginning of the end step, if enchanted creature's power is 4 or greater, destroy Arachnus Web.
6/8/2016 Activated abilities contain a colon. They’re generally written “[Cost]: [Effect].” Some keywords are activated abilities and will have colons in their reminder text.
3/14/2017 Arachnus Web’s triggered ability checks at the beginning of each end step, not just yours.
3/14/2017 If the enchanted creature’s power isn’t 4 or greater when the end step begins, the last ability won’t trigger at all.
3/14/2017 If Arachnus Web’s last ability triggers, but the enchanted creature’s power is reduced to 3 or less before the ability resolves, the ability will have no effect. Arachnus Web won’t be destroyed.
3/14/2017 If Arachnus Web enters the battlefield attached to an attacking or blocking creature (due to Arachnus Spinner’s ability, for example), that creature will continue to attack or block.
3/14/2017 Triggered abilities (starting with “when,” “whenever,” or “at”) are unaffected.