Should I use different sized RAMs?

[Shuvam_1]

My pc currently have a Zion DDR3 1333MHz RAM. I want to put another RAM of the same company and module but of 4GB size. Can there be problem in such system? Is there any solution to the problem (if any)? Or, should I buy two DDR3 2GB 1333 RAMs of the same company?

 

[CountMike]

Shouldn't have any problems but maybe partial or none of Dual channel.