What does a 64-bit system and 4GB RAM have to do with using 64bit timestamps?

32bit systems can use 64bit values without issue. In fact, even an 8bit system can handle 256bit values or even longer ones without issue.

The bittiness of a CPU and its address space have nothing to do with the length of usable data unless you end up with data longer than the RAM volume (and even then there's swap).