I ussed to have a 92 GSXR that had a moped battery mounted to the contershaft cover. I did live in Hawaii though. And that bike started instantly.
I suspect if your bike starts and runs well you could get by with one of these but why not get an 8 cell. I ran 2 of the 4 cells on a bike before due to space constraints. If you do this just be sure you understand the difference between parallel and series battery wiring.