Are all four SO-DIMM slots user-accessible in the 27 inch iMac (Late 2012)? I'm going to buy a new 27" iMac, but I think that the RAM on the Apple Store is really too expensive.

My plan is to buy with minimum (2x4GB) RAM and replace everything with 4x8GB.

Are all 4 slots designed to be user accessible, removable and replaceable?
The Tech Specs page on the Apple site states:

8GB (two 4GB) of 1600MHz DDR3 memory; four user-accessible SO-DIMM
slots
Configurable to 16GB or 32GB.

so yes, all four slots are user accessible.