I have just done this to my 2011 imac - mid or late, even MacTracker can't decide :) [br]
I took the two 2GB modules form one 2011 imac to add to the two 2GB modules already insitu![br]
At my first attempt I added the two modules to the lower slots - I got the 3 beeps every five seconds.[br]
So then I pulled them all out - reinserted the two modules I'd pulled from the other mac, into the upper [br]
slots and the imac chimed very nicely and everything worked.[br]
So then I added the two modules that already had been in that compouter, but this time I put them [br]
in the lower slots - and bingo, everything worked fine with now four 2GB modules in that iMac. [br]
Very Happy![br]
But a caution here - my initial problems could have been because I hadn't seated one or some of the [br]
modules properly in the first place - even though I'd was quite aware of that potential problem![br]
If at first you don't succeed - play around with it till you do! ...or don't! :)