one of the stupidest things Denon did when they switched to the retarded two-sided remote was to NOT put all of the input selector buttons on one side or the other. who's idea was it to put some of the inputs on the front and the others on the back?
anyway, there's no easy way to do it without opening the secret flip door. I highly recommend springing for a Harmony, even one of the older style denon remotes that had all the stuff on the front.
anyway, to your first question, you can hook up more than 4. there are five unique video inputs (HDP, VCR, TV/CBL, DVD, V-AUX) which are all assignable, then two additional audio-only inputs (CD and CDR/TAPE), plus of course the dedicated PHONO input. So that's seven unique inputs, five of them video, plus PHONO and TUNER.
You can also (as you mention) "stack" the inputs, so for example you could rename "VCR" as "VIDEO GAME", and then have the PS3 hooked to HDMI, the 360 hooked up via component, and the Wii hooked up via S-video, and it will "prioritize" and just automatically pick the highest-priority one that is turned on.