I thought the larger Ram board pairs were supposed to be in slots 1-2, so in your setup Riser A slots 1 and 2 would have the 4g boards, and Riser B, slots 1 and 2 the 2g boards, with the remaining 1g going to Riser A, slots 3 and 4. That would mean you should swap the positions of the 4's and 1's.