Limit data entry to one of four available combo boxes

Morning All!

I'm sure I've done this before but I can't remember how...

I have a form with four combo boxes on it, the four boxes all open the same form and return a record based on the selection from a different column from the same query.

To make it work smoothly I think I need to make it so it is only possible to enter data in one combo box at a time. I could also do with some error handling.

I've had a google, but I can't get the wording right to find the help I need. I would very much appreciate it if someone could point me in the right direction....

Thanks :)

Limit data entry to one of four available combo boxes