Installing and Upgrading Ram on a mid 2011 iMac (different size RAM)
My understanding is that you want to pair your memory sticks in order to avail of dual-channelling, and are uncertain if this configuration is compliant.

It is. Firstly, apple recommends keeping the factory memory in the same position as it always has been. Failing that, it's only important to keep the same-sized memory in the same DIMM slot.

As you have above, both 4GB sticks are in the zeroth position, and both 2GB sticks are in the first position. This (or the opposite 2GB in zeroth, and 4GB in first) is the optimal configuration.