A working tach is a must when setting up a prop. After you get the tach working run it wide open and make notes of your rpm, mph, and water pressure. Ideally you want to turn max rpm with a full load for best performance. I don't know what your motor is rated for, around 5500-6000 rpm. 4 blade will generally carry a heavy load better and handle better but will give up a few mph on top end.

I would check out the setups page in the bass boat central website. Tons of Info on prop selection.