Hypothetically... Can a restriction site on a DNA sequence be less than 4 base pairs?

it depends on what you're asking... does the question require that you're talking about a restriction site?

i haven't really seen any that are less than 4 base pairs

this has to do with the fact that restriction enzymes use a palendromic sequence to find where to cut

you can't really form a palendrome with 3, or 1 base pairs, 2 maybe but then really it's just cutting at what seems a random spot, in which case you probably wouldn't call it a restriction enzyme you'd just call it a DNase or endonuclease

you may try to look for endonucleases that only cut at

AT

TA

or

CG

GC

then try to argue that those are technically restriction enzymes

so hypothetically, you may be correct,.. you may be asked to find evidence for the enzyme though

i'm not really going to try to search for that but you can