CS3 Maimum amount of dowloads

I bought Photoshop CS3 and have a photography studio. I have 4 computers to do my editing on and when I tried to dowload the program to the last computer it says that I have reached my maximum amount of downloads or something like that. It won’t let me register it and it tells me that I have to de-activate one of them to register the last computer. Am I going ot have to go out and actually purchase yet another copy of PS CS3 to have it on that other computer??? Is there a discounted price or do I actually have to pay full price to add it to one more computer??? Thanks!