Use a wildcard search and replace:
Find: [A-Z]{3,4}

Leave the Replace box empty but put your cursor in there and choose Format > Highlight

This replace all will highlight all the instances with the last chosen highlight colour. Note that if your highlight colour is showing 'no highlight' then you won't see anything happen so you should highlight something so Word remembers the colour before you do the search and replace.

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Andrew Lockton
Chrysalis Design, Melbourne Australia