I don't think R&D is ever going to do a whole set of four colors. The number one reason being that they don't want tournaments where almost everyone plays 4 or 5 colored decks. They have many cycles of dual lands, but much fewer cycles of tri lands. Quad lands I think are undoable.
The only four colored card that could be common would be a 4 mana vanilla creature. Maybe, black and white mana cost, then blue and green activated ability to gain trample and flying until end of turn. But, there are already creatures like that in single color, why do it in four colors?
Enchantments, artifacts, lands, instants and sorceries are impossible to do at common and four colors. If you make it simple, 2 or 3 lines, it's too much colors for such effect. If you make it complex, it's rare, too complex for common or uncommon.
If the card is non permanent, I can only see it being: split card, or four modes to choose from, or kickers, the card costs two colors and has a kicker of another two colors to do a secondary effect. Either case, too much for common.
If the card is a permanent: double face, or pairs that care for another pair (say, a black and blue creature that cares about white and red creatures), or four abilities, one for each color, or two colors and two abilities of two colors (say, blue and white cost, one ability is red and blue, the other is white and green). Not doable at common and hard to fit for uncommon.
Hybrid mana highgest color count is three, one color plus one hybrid. Two hybrid and it becomes pure color identity crysis. For ex: white, green and blak have access to life gain. But imagine a creature that costs one triple hybrid and when enters the battlefield, you gain 1 life. It can be white or green, but black doesn't do that. If you add "opponent loses life", then it can't be green. That must be what R&D has found out when they tested triple hybrid.
If the card can be white and/or black and/or green, why not make it an artifact? Reaper King is both colorless and five colored, but that is only possible due to how close artifacts and five colored cards are in the color pie. Colorless is something that any color can do, while five colored is something that all colors combined can do.
The Nephelim were made before the popularity of commander. They just feel overdone, because their abilities are too much close to existing three colored or five colored creatures. They picked up an ability that is two colors and forcibly made it four colors by making it trigger or care about things that the extra two colors care.
Four colors, for the sake of "eveness", has to be a card design where there is either one effect that is four colors, or two effects that each fall under one of the pairs of the quad, or four effects. If you do three effects, then one color is going to bleed more than the other three, making it odd.