A UUID-4 has five groups of lowcase hexadecimal characters, the first has 8 chars, the second 4 chars, the third 4 chars, the fourth 4 chars, the fifth 12 chars.
However to make it a valid UUID4 the third group (the one in the middle) must start with a 4:
00000000-0000-4000-0000-000000000000
^