Hi. You will first need a AC-DC transformer since they are powered by Direct Current. Current from the wall outlets will be Alternating Current. This can be found at Radio Shack, or any electronic parts store online, etc.
Notice that each of your fans have an Amperage rating. You will need to find a transformer that can handle those amperage. It looks like the highest is 0.3 A or 300 mA. I am running two fans on a 500 mA transformer. You could do the same.
The next step would be to splice the power cable, and connect the FANS. IF you want all 4 to come on with ONE transformer, then you hook all 4 up. If you want 2 to come on when halides are on, and 2 to come on when they go off, then you will need 2 different transformers.
I hope that makes sense.
Best,
Ilham