ADressable memory is 4TB with 64 OS, the restriction are the used mainboards, mine has an limit of 64GB. The limit has nothing to do with the ability of 64 bit OS mappable memory area. 
However there is a builtin restriction on workstation os like windows 8, windows server enterprise doesn’t have this restriction (however to my knowledge there isn’t any mainboard which could hold 4 TB of memory sticks 
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