Ric I had an AT3 back in 87-89, it was the first notch machine I owned, the first time i took in the water it leaked, they were only waterproofed for 6 feet, it is a vlf machine, the coil wasnt weighted. It was basically a "water proof" version of the freedom 3. If you get Id be awlful care about getting too wet
I liked the machine though because you could vari the notch on it. one went from iron to above nickles the other control was set for above that. I dont know as much about the AT-4, it was supposed to have been an improved version of the AT3
hope this helps
dave+bobbie