Sorry no experience, but I knew poeple who had them way back when, and there are bound to be some still floating around. Single frequency means VLF; in todays parlance that is a no-no in salt water, unless you do alot electronically to overcome the salt effects - Garrett didn't back then.
As for performance, the AT4 was a good detector from what I heard, back in the day. As I recall the coil was smallish, 7" I believe and every old issue of W&ET had someone with one "cleaning up the beach". Depth is probably so-so by todays standards.
According to the manual, it had dual range DISC control which is cool, along with the p/b pinpoint feature - also cool. It would give a tone for accepted objects below coin and the famous Garrett 'Belltone' for coin range targets. The AT4 was also ahead of the Minelabs in that it used rechargeable batteries.
The o-ring seal is always the most important part of any wet detector. You dont know how the previous owner(s) took care of that, or if they messed it up somehow. The AT4 was not waterproof, by the way. Instead, it was "splash and dunk" proof. Mr. Garrett mentions it in his books that way. It is now long out of date, so there may be little in the way of service from Garrett on this oldie, should it be needed. Check with the factory on that.
Unless you can get a working model at garage sale prices, or already have one lurking around somehwere, you may want to err on the side of caution on this one.