Restricting a multiselect userform

[td]

My objective is to show the user a userform and they are to choose 4 stocks from a list of 17. They can't pick any more or anyless than 4 and they can't pick the same stock twice, if they do I have to send back a message correcting them. How do I go abouts doing this do I set up a loop of some sort?

Any input or suggestions would be much appreciated and useful, I need all the help I can get!!

Thanks,
td.