Yes, the max ram for that machine is 8 GB so you're good.
There are 2 ram slots. Hopefully, you have 1 stick of 4GB in one slot and the other slot is empty. In this case, you would only need to buy 1 4GB stick.
It's possible though that you have 2 2GB sticks (one in each slot) ... in that case to get to 8GB, you'd have to buy 2 4GB sticks.