I may not be online much longer, so I'll make some assumptions here....forgive me if I'm reading your post incorrectly.
2-channel & 4-channel describes an amp. 2-channel is usually used to run a sub (or set of subs) in mono. 4-channel are typically used to run front and rear speaker.
An amp sits between the head unit and speaker. if you're asking about a head unit, 2 and 4 channel has nothing to do with it. You can, however, get head units w/ multiple pre-amp (used to connect head unit to amp) output. Most will come w/ one, maybe two sets. However, since you said you aren't going to run an amp and sub(s), just pick a head unit that has the functionality you want.
Hope this helps.