Hello Guy!
The first complete specification of NTP, that is, Version 1, appeared in 1988 (RFC 1059) which provided simple symmetric and client server mode operation.
Version 2 appeared in 1989 (RFC 1119) and added symmetric key authentication using DES-CBC.
Version 3, which is the version that is most used today was first described in 1992 (RFC 1305) and has been systematically improved over the years. It introduced formal correctness principles, revised algorithms and broadcast mode . This is the default version that is available in most Cisco devices using the 12.X IOS version, and the recommended minimum version you should use.
Version 4 extends the support of NTP to IPv6 and is available on any Cisco device that supports IPv6.
I hope this has been helpful!
Laz