Ah, Beatles multitrack discussions...
So, let's look at the title song, A Hard Days Night.
Track 1 - Drums, Bass, Guitars (plural) - basically the band recording on Track 1
Track 2 - Vocals
Track 3 - Double-track Vocals, Acoustic Guitar, Bongos, Cowbell
Track 4 - Piano, Electric Guitar - The middle 8 solo, guitar at end and the piano in the opening chord all recorded at 1/2 speed in a lower pitch which when played back at normal speed became normal pitch
The open chord is spread across Tracks 1, 3 and 4.
So, if you want to make a "modern" surround mix, you'll probably want to move the vocals into the center front. Track 1 is in the front left and Track 4 is in the front right. So that leaves, well, nothing for the surround channels except maybe some of that good ambiance. If you don't use more modern "tricks" then that is what you are stuck with.
I could go up and down the A Hard Day's Night tracks and basically what you will find is very similar. With modern techniques bongos, cowbells (more cowbells!) and maracas could be separated. But that isn't exactly a fulfilling surround picture. Can't Buy Me Love gets a little more interesting but overall these are very simple songs with very simple recording techniques.
I think all of the AHDN songs used all 4 tracks of the multitrack and none of the songs needed bounce-downs (reduction mixes). Get "the band" on track 1, record the vocals on track 2, get the double-tracked vocals on 3 and then anything else that was needed went on track 4. Unless you can separate out "the band" on track 1 into separate tracks, you don't have too much to work with for a surround mix. Even then, it's still a very simple song instrumentation-wise.
It's not until Rubber Soul that the song instrumentation and vocals really get interesting and complex. It isn't until Revolver that the recording techniques got more complex (and yet Paperback Writer was recorded with 4 tracks total).
Andy