I have a 210 tall, so it's 6x2x29". I had an FX5 with an AC110 on the back. Kept it nice and clean. Then came across a great deal on craigslist for 2 more FX5s. Then I got an XP3, due to this site. So now I'm overfiltered.
go with another xp4 and you should be fine, there rated to like 265 each so as long as your not overstocking the hell out of the tank with huge south american cichlids you will probably be ok.