The current scan for BTTF is almost certainly 2K (or maybe 1080p).
Thankfully the arrival of Ultra HD Blu-ray means that film studios have an incentive to re-scan their catalog titles in 4K so they can put it out on the new format.
Some companies have even begun to re-release titles on regular ol’ Blu-ray with new 4K transfers. Doubt that’ll happen with BTTF, though.