MOSFET current question
MOSFETs can handle currents, but there is always something to engineer around them. Normally you have to check power dissipation in your working conditions (caused by current and switching according to your system) and see if the MOSFET can dissipate it alone, or may require special layout or even heatsink. So when they state 195A, they may mean "with water cooling attached". Of course, this is not what they will write, they will just provide all required information so you will be able to check by yourself.

Take a look, this is your MOSFET's datasheet. They say Rdson is 1.5mR, so if you don't switch the resistor and current is 100A, the power dissipation is P = I^2*R = 100*100*0.0015 = 15W. You definitely would need serious heatsink for that. But for your 4A the power is 2.4mW, so it will stay cool.

One think you should know- your motors are probably brushless, which means you can't just switch them on and hope they rotate. You need a controller, either off-the-shelf or designed by you. And since moving a brushless motor is not trivial- go to ready made products first. Don't waste time.