Your grouped seems even more geared than mine as our DPS are mostly DL with 1-2 pieces of myth and 1 coil accessories, the only geared DPS is our Dragoon.

For our phase 4, each tank will take 1 rook, our DRG will focus DPS on 1 rook till it died then move to the next, the rest of our DPS, 2x BRD and BLM/SMN will aoe everything. I am one of the Bard so I will provide Mage Ballad for our tanks to use flash so they can hold agro on the spiders as well. For us the rooks goes down pretty fast, while spiders goes down slightly after phase 5 starts.

I find it strange that your OT dies so fast though, our OT who actually main a WAR sometimes goes as PLD with only DL+relic doesn't die in phase 4. Maybe TirionCrey suggestion where a DPS takes a rook and OT just kite the spiders will work since your OT have problem handling a rook and the spiders at the same time.

BTW it seems to me that your group DPS is a bit low during phase 3 dread, you should be able to kill it before phase 4 even starts. My group save all our cooldown to be use during phase 3 and I'll provide Foe Requiem for our BLM/SMN. Before we start using food we kill it just as phase 4 starts, now with food, we use dodo egg, we can kill it before phase 4 starts.