With just 600mA peak current, an LM317 on a heat sink should be fine for this application. It should be positioned between the TIP32 and the fans, so that it turns on and off just as the fans do in your drawing in #16. It will be configured as a standard voltage regulator - see the data sheet. The variable resistor can be a 1K with a fixed resistor of 470Ω in series with it. This will prevent the set voltage from dropping below 4V. Use 680Ω for 5V, or 820Ω for 6V minimums. Do some testing to determine which minimum voltage works for your fans.