Ok, you have two upgrade options for memory (RAM). 12GB or 16GB. Your current system has only 4GB, so an upgrade to 12GB will obviously be better. But 16GB is even better! Depends on how much you want to spend. 

Here’s the story: Your computer has four slots for RAM (different than Zoe, who only has 2 slots). Each slot can hold either 2GB, 4GB or 8GB of RAM (or be empty). However, the TOTAL amount of RAM from all four slots cannot exceed 16GB. 

This is your current set up: 

Slot A: 2GB
Slot B: 2GB
Slot C: Empty
Slot D: Empty
TOTAL RAM = 4GB 

Option One: Upgrade to 12GB of RAM. $47.99 on Amazon. 
Slot A: 2GB
Slot B: 2GB
Slot C: 4GB
Slot D: 4GB
TOTAL RAM = 12GB 

Option Two: Upgrade to 16GB of RAM. You will toss out the two cards of RAM currently in there, and replace them. $95.98 on Amazon. 
Slot A: 4GB
Slot B: 4GB
Slot C: 4GB
Slot D: 4GB
TOTAL RAM = 16GB 

For option one, buy just one of the item below (4GBx2). If you decide for option two, order two of them.  

Here are the instructions as to how to install the RAM once you receive it. At the top of the page, click on iMac (21.5 inch, Mid 2011) to take you to the pertinent instructions. 

And of course call with any questions!