Cool! Please drop me an email if possible, I would like to explore how the reduced bits in the registers affects Redis ability to estimate very very high cardinalities: imagine millions of events / second running for years. The problem with improving the current Redis implementation is backward compatibility. I made the error of representing the HyperLogLog in Redis as a string instead of using an opaque data type, so it is not possible to do conversion-on-load of old data, but there is to convert at the first access.

If there is a version annotation it is very easy to convert the old HyperLogLog to a 4 bit register representation. I will send you an email ASAP.