I'm begging to zero in on what I want to get to replace my rather old macs. I'm looking at the 27 Inch right now. I know the RAM is accessible for self-upgrades. My question is are all 4 slots accessible? My plan would be to get the base 8 gigs (2x4gb) then if I decide to add, fill the other 2 slots, and then if needed down the road, replace the 2 4gb's, but if I would be locked into the 2 4gb's, I may need to rethink that. Thanks.