If these 4 x 6 are for rear speakers, you can forget about them : they will do nothing good for you. Just try to play your system with these off, and hear what it sounds like. Must be better, at least for the stereo effects!
You have powerful amps for your speakers, you can have 2 sets of component speakers with 200 watts rms to each speaker. Focal K series are great really great (I run a set of 165 KP and I loooove them!!!), but expensive.
I've heared good things about the Alpine type X pro, and lots of people seem to love Image Dynamics speakers.
The RE xxx ? Never heared them, they look awesome but some reviews are not so good about them.
Why don't you go listen to the most sets you can and tell what you liked or not, so that people here can help you more.
You can chose a good set of midbass speakers on a 400.4, and a set of comps on the other amp : think about it.