I dont have powder blue4s but I do have 3 tangs. I have heard and read that the blues are super aggressive and kinda hard to keep. Just what Ive picked up here and there.
Now, I have a blue hippo, yellow and tomini tang in my 125g currently. I purchased them awhile ago and added the yellow first and then the other two. The yellow was hard on them both for a few days but they now are all friends and swim together as my little tang gang lol. For the reason of the initial aggression i would suggest adding your tangs all at once first off. Secondly, I purchased the blue hippo with the intention of upgrading before he got too big for the tank which he kinda is already. I was certain that I was doing an upgrade though, I mean absolutely certain and to that effect there is a 180g sitting in my living room waiting for water.
If I had any doubts about the upgrade I wouldnt have gotten the hippo. I babied him from the moment I brought himhome. He was qt'd for a few weeks in a 55g with the tomini and watched like a hawk. I have been very lucky with him, very lucky. No ich signs ever and he got huge and fat. It can be done, but it takes time and effort to keep them happy.