I never had a 4th gen, but I just replaced my head unit a couple weeks ago.
Pop the plastic cover off with a flat-head screwdriver--there are about a half-dozen snap-in tabs, so be careful. Then just unbolt or unscrew everything. The whole unit slides out. There are brackets attaching the cassette deck to the HU. About 4 screws on each side--save them.
After taking it apart, it looked to me like the original bin was broken off so that the cassette deck would fit in there. It was a dual-purpose bin/bracket. I bought a new head unit from Crutchfield and they gave me a replacement, but it's not as big as the Honda bin. You can probably just buy one from them if you're desperate to cover the hole.
Here's the pic:
edit: sorry the pic doesn't display--cut and paste the address and you should be able to see it. DAMN YOU WEBSH1TS!!!