Donna,

Question no. 1 is, is there any indication that the walls need weeps? Are the
walls bowed or otherwise distressed?  OK, they probably don't have them
because the person who built the wall was too stupid/lazy/cheap to put them
in, but there could conceivably be a reason why there ain't no weeps.  Are
the walls reinforced? Is there gravel behind (on the uphill side of) them?
If there's no distress, maybe (yeah, sure)
somebody put in perforated pipe to drain the backfill.

My guess is you should put the weeps in 32" or 48" on center, probably in the
lowest 2 courses of block. (This would give you holes 16" or 24" on center in
a W pattern). 4" diameter holes are going to be too big, unless this is in
fact a dam and you're trying to create spillways, or squirrel condos if these
are really retaining walls, but even then 4" would be too big. I would drill
3/8" or 1/2" or holes  in the mortar joint where the head (vertical) and bed
(horizontal) joint meet, so that when water passes through the wall, it
doesn't have to fill up the cells before the overflowing through the outer
section of the weephole.

Now let's sit back and see why our colleagues think I'm completely full of
shit (about this; let's be specific), and why your weepholes at 4" are too
small, even if you spaced them 3" on center.

Ralph