Some of those replies show a difference in brands, apparently for identical spec chips -- where one person swapped his MB and MM boards and both worked.

Turns out that my iMac has 2 x 2 GB, also just 2 slots..
Will likely settle for the 6 for now anyway; by time that extra-50% goodness wears off, the next 4 GB + trade-in creds will be a bargain.
Paying a tad more at iFixit is OK because that's how they pay for their superb How-tos and they guarantee at least the 6 GB kit.
(If the problem on iMac is also firmware, mayhap that too will happen by the time I need it.)
Ditto for you if MM problem is firmware, etc.

Thanks for the sleuthing.

A.