At this point it seems more like the community idea of how healers function and should be balanced is the biggest obstacle.
The community seems deadset on wanting a comp with one "shield healer" and one "regen healer" and having a 50/50 split for shield and regen healers so the 4th healer must either be flexible like AST or ASTs stances need to get scratched - and all for this misconception that you always need on of each when in reality, Diurnal is the prefered stance outside of early prog regardless of your co heal and Neutral gets rarely, if ever, used during a fight and mostly during transitions.
WhM/ SCH comps mainly utilize direct heals and regens while shields (even the one from Seraph because of mandatory two weaves) rarely get used.
For AST/ SCH, Diurnal is prefered not because they wouldn't be able to happily spam their GCD shields but because Diurnal offers much higher raw healing and is more valuable outside early prog. Some speedrun niche cases work with SCH/ Noct, but that's highly optimized gameplay and GCD shields have nothing to do with it.
AST/ WhM is Diurnal again for AST even though PF loves to enforce the rule to go Noct with a WhM although it generally makes things harder for both healers because you're missing out on a lot of free healing for no good reason.

What's more important when it comes to healer balance is the rhythm each healer has and the opportunity cost tied to their heals.
If one healer has high opportunity cost, it doesn't matter that their direct heals, shields or regens are strong - they will rarely get used to aim for better combined dps. The healer with the lowest opportunity cost and/ or more flexibility generally does most of the heavy lifting and the other healer has more freedom in planning their forced dps loss around times when they suffer least from it, like timing Raptures and Ruin IIs with forced movement that goes beyond what slidecasting and prepositioning would cover.
Sacred Soils is one of the strongest skill that never gets used unless absolutely neccessary because a weak free heal is better than a costly strong heal when both would suffice to survive.


There are a myriad of ways to design interesting heal skills (delayed, grows stronger/ weaker with time, consumable, stackable buffs to consume, grows stronger/ weaker with distance, jumping heals, small radius, stationary, multi-target small radius, copied healing etc.) that function well with every healer.
If the 4th healer functions well with the other 3 in the way that it doesn't "punish" some by forcing constant dps loss from both healers (like SCH/ WhM currently still does to some extend) and thus making it undesirable, it doesn't matter if they lean more towards shields, regens or good old direct healing.
Balance would definitely survive it but I'm starting to think part of the community will blow a fuse should we have "too many" healers of one type.