[QUESTION] LTD ex-401 truss rod wrench doesn't fit?

I went to set up my new axe I bought last year and the tool that came with the guitar doesn't fit the truss rod(!)

It's a 4mm hex key, and it feels too small, tried a 5 and that's too big. As far as I know there's no imperial size which sits between 4 and 5mm. What the hell ESP?