If you really can't think of any more, you can leave off the fourth prompt, but the person who'll be writing for you will need to have a choice, so you do have to have your three. They don't have to be very complicated.

Alternatively, if you just want to write, you can skip signing up for the exchange and pick a prompt or two from the community prompt list - it's everyone's fourth prompt, and it'll go up on the comm the same day exchange prompts are sent that. :)