I have a MacBook Pro Unibody with one memory slot with 2 GB and I want buy a new 4 GB memory for my second slot. Can I have a problem using 2 GB + 4 GB?
Try Crucial.com 's Mac Memory Advisor Tool.
It will give you suggestions based on your particular model.
Intel Mac's no longer need matching size RAM modules, but there is debate about whether it is still advantageous to install RAM in identical pairs ( possible gain in performance by taking advantage of Dual Channels ).
More info here.