This isn't just designer conceit; players want a four color set. And it's fun to explore the design needs and tricks.
Players claim to want all sorts of things. Enchantment Block, for example, or more Cipher cards of higher power level. Or more storm. It doesn't mean that the resulting gameplay is going to be positive.
You can absolutely make some four-color designs work. Snowfield Rescuer works reasonably well - though it's also pretty unnecessary. You could make this card in mono-green. Green gets 2 mana 2/1 flash creatures. White doesn't need to be here at all either. I can see the argument of making sure that more decks get to play it, hence the hybrid triggered ability, but it doesn't really work like that - because you need to play this in a deck running blue and white. Additionally, it doesn't do a good job supporting the 4-color-identity on its own, as it reads more like a card that is either "Esper or Bant" - just like the hybrid cards in Fate Reforged read like cards that are either allied color or enemy color - which fits the bridge-between-two-sets theme. There's a reason they didn't use hybrid in Khans.
Not to say you should stop the project or anything, it's great to explore weird new space. Go for it. Just want to bear in mind these challenges as you search for the perfect solution.
I feel for-color design is an example of "just because you can do something doesn't mean you should".
If it was going to happen, I think the best suggestion I've heard so far is the "Meeting of the Guilds" idea from ravnical. 4-colors is hard to think about, but 2 guilds is easier to understand, especially if it uses both guild mechanics.