I have a very complicated spreadsheet that many different users are filling
out. It is a complex spreadsheet that I cannot change. I've got a macro to
help them input data into this spreadsheet. I need a little help with it.

How would you create an input box that checks to see if the data entered is
correct? Below is the code:
ActiveCell.Value = InputBox("Which BLI would you like to
enter?")

What is needed is to check the BLI number entered, which must be a 4 digit
number, i.e. to verify if it is long enough, and if it is one of the 22
allowable numbers. Normally, I'd use a data validation for the cell in the
spreadsheet but the users are "getting lost" in the spreadsheet. Any help
would be appreciated.