Having used both recently, bebop, I think I can answer this one...
Probably the most significant decision you have to make is, do you want to overdub? The H4 can do this, the H2, not. So, you COULD put down the backing first, and then overdub the solos (but you could do this just as easily using the Sequencer)...
Do you intend to record audio on the mikes? Maybe some friends come to jam, you want to stick up some mikes to capture it. The H2 has a far more flexible mike arrangement (TWO sets of stereo mikes, with different pickup patterns, and capable of recording in front AND behind the unit at the same time (in surround, if you want!), so you just stick it down in the middle of you, and jam away...
The H2 has multiple folders to record into, the H4 just one. This can make a difference if you want to have some files for playback, separated from the files you have recorded.
The H2 is a much smaller, more compact form factor. The H2 will time-stamp all recordings, so that makes searching for something you know you recorded last Tuesday afternoon in the midst of a bunch of other recordings.
The H4 has some mike modeling processes, the H2 not. I never found them to be that good, and you can always do this after the fact on the laptop, anyway.
The H4 has XLR and 1/4" inputs, the H2 just 1/8" minijacks. No big wup, unless you want to use some external mikes, then you'd need some adaptors (and possibly a pre, not sure if the H2 has enough gain on the external mike inputs for a lo-z mike).
Overall, I could have bought either. But the smaller form factor, better mikes and no need for overdubbing made me pick the H2.
Anything more technical than this, you can check the website out
http://www.samsontech.com/products/productpage.cfm?prodID=1916 http://www.samsontech.com/products/productpage.cfm?prodID=1901&brandID=4 Hope this helps.