Regex - Partial Mask

I am trying to do a find and replace in msword

problem:

I need to search for the following format

REF NO: 84542654
REF NO: 21364589

i need to replace the first 4 digits in the number with * so the iuput should be

REF NO: ****2654
REF NO: ****4589

I cannot seem to get this working

I have tried the following

Find - REF NO: \[0-9]
Replace - ****

I cannot figure out how to specify to only replace the first 4 digits.

I am happy to use VBA code via a module as an option

can anyone help?

I have been pointed in the direction by somone

the answer is

find what - REF NO [0-9][0-9][0-9][0-9]
Replace with - REF NO ***

s/REF NO: \d{4}/REF NO: ****/g

Modify to match whatever language you're using.