I am trying to build the circuit in the following link.

http://www.elecfree.com/electronic/wp-content/uploads/2007/07/charger.gif

The only problem is this was designed to output 12 volts and to charge 8 AA (1.5V each) batteries in series. I only want to charge 4 batteries, so what would I need to change to accommodate this?

 

http://www.elecfree.com/electronic/nicad-battery-charger-by-fet-irf9530/

Also here is a little write up about the circuit.

 

http://www.vt52.com/diy/myprojects/other/charger/charger.htm

Try one of those chargers.

 

If you already have the components all you need to do is to connect the two transformer secondaries in parallel, rather than series as at present. This will halve the output voltage for 4 bateries in series rather than 8.