I have Sonar Professional.  It comes with 4 modules.  It can only operate 4 modules at the same time.  My question is, can I add/buy the concrete limiter module and use it with sonar professional as long as I'm only using no more than 4 modules at a time?  or do I have to have Sonar Platinum to do that?  I don't want to purchase the concrete limiter and not be able to use it...

You can buy any ProChannel module that's available separately and you can use more than 4. You'll just need to use the scroll wheel. 
 
Also, you now have three new modules thanks to the addition of Style Dials. They are very useful. Check out the Ipswich update.

Thanks!