Go with the 2 4's. I use 2 4's in my 65g and it's still not enough for my tastes.
You just have to get creative when it comes to placing them and directing them.
I have mine on the sides, bottom back, pointed up across the back. I get nice
flow bouncing off the back of the rocks and coming to the front of the tank.
It keeps everything nice and clean back there. Get creative, you'll find that you
can really push a lot of water through a tank, and keep everything happy.
I have a koralia 3 and a MJ 900 w/mod in my 90 gal. I would go with a koralia 4 if I didn't have torch coral which is getting heavy flow already. I also have a ton of flow created from my return so that would factor into your flow requirements.