Adding Memory to an iMac

I bought an iMac a couple of weeks of ago. I wanted to get it at the Apple Store rather than order it online. They only had the i5 model with 8GB of memory in stock. I was hoping to get 16 GB since I like to have many programs open at once.  I got my MacBook Pro with 16 GB, and I have never had memory issues. I asked what it would cost to get an upgrade at the store and it was $400. I passed. 

Today I looked at the About this Mac entry on the Apple menu, and it told me that I had two 4 GB dimms in two of four slots for a total of 8GB. I had expected the computer to be limited to two slots like the MacBook Pro, since Apple uses some laptop components in its iMacs. 

I can get 8 GB in two dimms from for $99, and the iMac has a memory access door, so I do not have to take the computer apart. I will order it today.