Use different memory capacity RAMs in mac mini
You need to find out what RAM your computer is capable of handling before you just start putting RAM into it, even if the speeds are the same, it's never to much trouble to be careful and do a bit of research to make sure things go smoothly. 

Thankfully there are a lot of websites that have already tested these things for you so that you don't make any mistakes.  I usually go to:

http://www.crucial.com/store/drammemory.aspx

They have a system scanner service that will scan your system and check your settings, it will redirect you to a page where you can buy RAM, you don't have to obviously, but it will show you all the RAM configurations that your system supports.  It will let you know the max RAM capacity, if you don't see 4GB as an option, then it doesn't support it. 

If you don't want to use the scanner you can also just input your make model year and cpu to get the same results, though the scanner is usually more accurate if you aren't sure which machine is yours. They do tests to make sure the RAM is compatible and list all the speeds and configurations for the RAM so you know which ones to get.  Make sure everything matches up before hand.

Knowing these things will help you find out if the RAM you have or are about to purchase will work so that you don't waste your time.  I hope this information helps you out.