Suggestions on how to do this

Here is an idea that I have been thinking about for a little while for use on one of my sites. The concept is simple, but I cannot think of how to accomplish this. Perhaps someone here can suggest something. On one of sites, the users must choose 1 of 4 banners before their entry can be submitted. Currently, there are 20+ banners in the display pool. I am choosing and displaying these 4 banners randomly. What I want to do is to limit the times a banner is shown to a user for a given time. ie...user should only see banner 1 twice in a 4 hour period. Has anyone already come up with something similar? I have seen it used in banner programs, but can't figure out how they did it. Any suggestions on how to accomplish this? TIA CDitty