I was thinking a similar thought that 4096 seemed quite magic (although it seems to be chosen based on research) and that RB perf could probably be tuned based on the workload
I don't think 4096 is arbitrary. It's an array of 16bit integers, and 4096 * 16 = 65536. So 4096 represents the boundary point below which an array of integers uses less memory than a traditional bitmap.
