If you want to upgrade the amount of RAM you have, make sure that 1) it is safe to access the RAM without damaging the PC, 2) That the PC can support more than 4GB of RAM
It ought to be able to, but if you are trying to populate 2 DIMMs, then make sure there are two DIMMs. Otherwise, you may have to buy a whole new stick of 8GB RAM. Also make sure that you are using laptop RAM and not desktop, since they are different sizes.
As for what your PC would do if you have two different speed RAM modules, it is actually very simple. Computers don't like a variable like that and it should lower the speed so that all DIMMs operate at the same speed. So if you have 1 2400MHz stick and 1 2133MHz, they would both run at 2133MHz.