This is the default or base UUID for all characteristic IDs if not set by the factory.
The default is 00000000-0000-1000-8000-00805F9B34FB
If the manufacturer is giving you only 4 digit then it is assumed that you need to use the default UUID. It starts with 8 hexadecimal digits and you need to add your 4 digit hexadecimal number as the second part of it. So instead of all zeros it would be 0000 plus your code FFF4