To follow up a bit on Dave's post, it's important with these devices to get your head around the fact that channels and tracks are different. The 424mkii has 4 channels that you can plug mics into (1-4 on the left side of the machine). Using the pan control for each CHANNEL, you can send it to either the left buss, the right buss, or both (pan ctrl centered). THEN, bassed on those pan controls for your input CHANNELS, you can select which track to record to by the little sliding switches under the digital readout. Track 1 and 3 will pick up any CHANNEL panned left (in the Buss L switch position), and 2 and 4 will pickup any channel panned R (in the Buss R switch position). So, you could plug 4 mics into CHANNELS 1-4, pan them all hard R, and record them all mixed together on track 2 OR 4. OR both, if you are so inclined.
Now, back to your original question, using an output from your mixer, you'd be best off using the appropriate cable to simply route your mixer into one of the CHANNELS 1 -4, and proceeding the way I mentioned above. If you want a mono drum mix, swing the pan for however many outputs in one direction or the other, and switch your Track select switch accordingly.
It took me (literally) years to get away from the direct recording option (the other switch selection), and it does open up more mixing options where needed.
Edit: Regarding the sub input, I tend to use this one as a way to add one more track (typically fx from a cd, or something) when I am mixing down the whole multirack mix to something else - computer, minidisc, etc. I don't use it to add tracks to the 4 track tape. YMMV.