I want to stay under Synology's limit of 6GB of ram, but I am unable to find a dual-rank 4GB module for the drive. The ones that you have listed are single rank, but you seem to recommend dual-rank for Synology RAM purchases.
Yes, you may not find them as easily. Usually, they only produce dual RAN RAM, when those memory chips do not fit on a single side. Usually with bigger capacities. Otherwise, you will more often see single-rank RAM.