I'm working on a shield that caters to the aquarium and indoor growing communities too.http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1258412419
If you're interested, maybe we can put our heads together and create a full package. I have a lot of experience driving fans as well. Between your Ph shield, my power LED shield and maybe some others, we could make for a complete setup? What other things need to be controlled? Pumps? Air? Temperature?
The fans used in computers have a number of possible uses for the extra wires. These are the most common:
o PWM speed control input. Usually handles any voltage between about .7V and V+
o Standard or open collector tachometer output. Usually 4 state changes per revolution
o Rotor lock/min speed alarm output. Changes state if the fan stops spinning or spins too slow
A 4-wire fan usually has the PWM and OC Tach on the two extra wires. That is the most common configuration I've seen. Rotor lock and min speed alarm is used in web servers and such. Standard tach is not very common anymore because the OC tach is way more flexible.