You should know that everyone here on this site will always tell you the same thing... to not mix different RAM or RAM bought at different times.
Well, from my experience... I've mixed up RAM before and never had any problems so don't be frightened by "scare tactics"... you will surely receive them.
Just make sure they have the same timing and also make sure you keep the receipt and the store has a good return policy. If you run into problems you can always return them, right?
But to answer your question... YES, you'll have 8GB of RAM if you installed another 4GB.