424III vs 464
The 464 has 4 single channels and 2 stereo channels, which I think are refered to as 5/6 and 7/8. If memory serves me right on the 464 you have to rely on panning to assign what track the stereo channels are assigned to, but when you assign them to a track they become mono. There might be anouther stereo channel on the 464, I cant remember.
The 424III has 6 single channels and one stereo channel that gose to the master mix and can be assigned to the master section but not a single track (1-4). Channel 5 can be assigned to track 1 or 3 and ch 6 to 2 or 4. So you could use the stereo ch to bring in a CD/Keybord/Or even a outboard mixer, but I'm not sure what happens when you are multitracking and not just using the 424III as a mixer.
One sort of cool thing I'm doing with the 424III, is using it as a pa mixer and recorder, at the same time.
I run channels 1-3 with mikes for vocals and pan them hard left. I then run my left main output to the amp/speakers, and I have the pa set in mono.
Then I use channels 4-6 panned hard right to record with. What you can do is mix ch 4, 5 and 6 to track 4 and you can get a nice practice recording (in mono). Ive been using a single omni mike in the ceiling, so we could listen to our practices.
You could also add mike or line in from the pa to inhance the vocals a bit, or even put a third mike over the drums if your amps are far away etc etc.
Have fun with it, rock on baby!