8x8 resource blocks are how UO handles resources that respawn. Resources don't restock in a tile/tile basis. So when you just fish in 4x4 blocks around you, you are depleting all possible resources by only fishing in 4 spots!
Actually it works out pretty close actually. If you fish one 4x4 square to being empty, then the other 3, you are really close to having the resources replenished. Trust me, I have a couple fishing scripts out there and have been doing the fishing thing for just about the beginning of my scripting life, so I've tweaked it a bunch. It's all come to successful fishing attempts/hr so if you over-fish an area, you'll waste a lot of time in that same area knowing you won't fish anything up.
So I guess I didn't explain correctly. When I meant fish "once", I meant fish in one area of a 4x4 block and exhaust the fish. That effectively fishes out the whole area and then you just shift N,S,E,W to the next 4x4 block. You can effectively fish in one place constantly and never run out of places to fish and you'll only fail to fish 4 times. Try it, it works...

Here's a link to my fish hunter:
http://www.scriptuo.com/index.php?topic=133.0The other one is for the Elite members and that one handles collection, archery and slayer books, ancient AOS farming, etc.
Oh, and there are places in UO where you can use this 4x4 method off land so you don't even need a boat!