I'm looking at upgrading the memory in my mid-2011 iMac. Is there any problem with sticking in 2 4GB modules alongside the existing 2GB modules?
You shouldn't have any problem doing this. According to this Apple kbase article, you can install any compatible 2 GB or 4 GB RAM modules in each slot.
Only in certain Mac Pros do you have to worry about specific memory ordering.
The iMac (Late 2009), iMac (Mid 2010) and iMac (Mid 2011) have four SDRAM slots, each of which can accept a 2 GB or 4 GB SO-DIMM.