I wont repost my party setup as its already in this thread a page or so back, but in a 4 DD party with WHM and BRd as support, the BRd and WHM each pick 2 DD and can fully support them np. DDs them selves should bring echos, eye drops, antidotes, holy waters. So the 2 support should only need to cover Erase and the WHM on stona.
Or if you do what we do, you can have one SCH that is roaming and able to just AoE Stona any party that needs it.
The only time I see issues on trash mobs is when 2 mobs with goofy/nasty AoE get too close to each other, and healers or melee take Aoes form both mobs. Outside of that, Mobs really dont spam TP that much if each of your MNKs knows when to penance, and your only putting 4 melee on a mob at a time.
My perspective is somewhat diffrent then a lot of others since we always have 33 (+3 mules for the 2 week cycle) we have to sit people each run. So it may be a little easier over all for us in some aspects. Only thing I could think that may cause issues with supporting 4 DDs is if for some reason your BRDs are /nin which is really a useless sub for BRD in Einherjar imo. As far as our DD go, every one of them is /nin so they are all tanks, which cuts down on healing needs a ton.
The other thing that may help is when you build a consistant allaince, I always have the exact same BRD + WHM and 4 DDs in each of my 3 main DD parties. They all know and are very good at working with each other by now, The healers know exactly who is going to have the most hate, and it makes things really smooth.
As far as testing the 6 song rotation, i suggest it actually on a tier I. You have a lot of room to play with time, and all the bosses except the Buffalo are easily zerged exactly where they stand. (Its possible you can do the buffalo where it stands also, but we plan to pull it to the back little tunnel so it cant knock back, but we are yet to have it as a boss)
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