Power switching circuit design

Hi Andrew,
first you need to design a comparator to your needs, see for example here: http://home.cogeco.ca/~rpaisley4/Comparators.html
Note that the LM393 has an open collector output which means, it cannot directly turn on the MOSFET (which needs a positive gate voltage to turn on).
Depending on how fast you have to switch the MOSFET, you can use a pull-up resistor at the output of the comparator and connect the output/pull-up to the gate of the MOSFET. This will turn on the MOSFET a bit slowly, but turn it off very fast.
If you nedd to turn on and off both fast, then you should add an active totem pole output stage after the Comparator. Or use an OpAmp instead, wiring it as a comparator. Thus you will have enough current to quickly charge and discharge the MOSFETs Gate, switching it faster.
For the gate drive 5V should be sufficient for 4 A Ids.

Regards,
Harald