I am not very familiar with MOSFETs and would appreciate some suggestions that will enable me to vary the speed of a 12V fan that draws a maximum of 4A.
I have heard I have to use n-channel logic level MOSFETs, would the following two models do the job or are there better options?
Normal good old power mosfets need about 9V or 10V at the gate to switch it on.
So that's why you need a 'logic' power mosfet. They are designed to operate with logic levels from a microcontroller or a Arduino.
You could also buy any n-channel logic power mosfet from Ebay.