Simplest way to control the speed of 4 identical fans
As peril brain already pointed out in the comments, a simple resistor in series is the easiest solution.

Note however, that speed regulation with a resistor is poor if you want to achieve very low speeds: fans require more voltage to start turning than to keep turning. (They need to overcome static friction.)

Most intelligent fan controllers therefore start up the fan with a "burst" of higher voltage and then reduce the voltage. Obviously this in not something you can do with a simple resistor.

I had fans that could not be made to turn slower than half speed with a simple resistor. Else they would not start. With others this was no issue at all. It really depends on how much friction your fan has.