This page shows the correct RAM for your model and, if you scroll down, there is a link to videos that show this is an easy do-it-yourself job.
You have four RAM slots. Likely two are occupied by the 4G of factory RAM (two 2G modules). If you add two more 2GB modules in the empty slot,, you'll have a total of 8G. You'll note you can add a lot more.
Data is stored on your hard drive, not the RAM modules. When you turn the computer off, all the data held in RAM is written to the hard drive so you will not risk data loss in upgrading your RAM as you won;t be touching the hard drive.
This should be your model:
You can go to 16 GB; mine is the same model (except that it's a 27") and I chose to go to a total of 12 GB - added 2 x 4 GB from OWC (the vendor suggested above). I'd suggest going with adding 2 x 4 GB because: if you want to go to a total of 16 GB at some point in the future, you only need to replace the two original sticks to get to the 16 GB. If you add 4 GB now, you'd have to replace all four at that time..... just a thought.
No effect on the warranty and, as long as you keep the original sticks where they are (matched horizontally) and add the other two horizontally, absolutely no problems. As I said, it is the exact configuration I have.
So, if you add 2 x 4 GB, you'll have:
2 x 2 GB (orig) plus
2 x 4 GB (new) for a total of 12 GB
Later - if desired - take out the original 2 x 2 and replace with 2 x 4 for a total of 16 GB.
As you can see here:
2 x 4 GB is $46.88 and 1 x 8 GB (same total GB) is $125.99:
I'd stick with 2 x 4 GB - the 8 GB sticks are very expensive.