I'm going thru a couple hundred 4 track pre records right now and I find that lots of them have very noisy backgrounds - hum and multiple layers of hiss. If you listen right from the head of the reel you can hear the quiet unrecorded head of the tape, and then the duping machine starts to record and it usually ain't pretty. Doesn't get much better when the master starts to roll. Chances are 120Hz was on the running master, as 120Hz picked up by a slave while recording at 8X would play back like 15Hz. You could probably determine the source by listening to the very start of the tape as I have described here.
Add edge curl and the associated left channel drop outs, brittle tape, occasional dull highs and channel imbalance - I can't say that I find the major fraction of 4 track pre recorded tapes particularly exciting to listen to. But when you get a good one it can be quite a pleasure.