Here is what you want to keep in mind: Say that the transistor hfe is 200. Say also that each fan draws 300mA So, you need a base current AT LEAST 300/200 = 1.5mA. This is true for max power. For minimum power you want to do some tests and measure how much current the fan draws at the requires voltage. Lets assume that the current is 1/10 of the max current, so it is 30mA, or 0.15mA base current. For 4 fans, you need at least 4x0.3=0.6mA current.

As a general rule of the thumb, the voltage divider must allow at least 5 times the current required for the base, that is 3mA.

I suppose you're talking about your circuit??? Its kinda hard to calculate this circuit - its a funny connection. But anyway. When the potentiometer is at max, then R1 becomes 0 and R2 becomes P//R2 (// means parallel). Same, at min, R1 becomes P//R1 and R2 becomes 0. So you do the math and find the values.

Using my circuit its much easier. The current through the voltage divider is always constant, its the sum of R1+R2+R3 under voltage. Roughly, its 12V/15KOhm = 0.8mA. As you see this is too small for your application. So you want to have a smaller potentiometer in order to further decrease R3. So, since the V_D calculation is linear, we get a ratio of 1/5, the potentiometer becomes 1KOhm and R3 becomes 2KOhm, total resistance is now 3KOhm, so the current is 12/3K = 4mA.

This is how you want to do your calculations.