If it is one of the models that support PWM you would use the fourth channel, if not any of them will do. Just remember channels 1 and 2 are thermally connected as well as 3 and 4. So if you have decent loads on channles 1 and 2, put the pump as far away (ie: channel 4) and vice versa.

For a normal pump with analog voltage control you would use this cable:
http://shop.aquacomputer.de/product_info…roducts_id=1622