Here is the answer : If your laptop has a free RAM slot , you can upgrade , but for my ASUS (R556L) it's like this , if i put in another 4 GB's of ram , it wont sum the on board module with the external , its becouse the on board module is soldered and it heats up faster and is inefective in high usage ( like gaming) , but when you put in the 4 GB's extra what you get is the same 4 GB's memory but slightly better preformance.
So if you wish to upgrade , go for 8 GB module and it will be like you have 8 GB's of ram installed !
But before you do its critical that you unscrew the back of your laptop and check if you have a free RAM slot ! And check what voltage your cpu supports .