This is a guess mind....but 4.2mm
(Cause I've run into this size before)
Good fergin luck finding a 4.2mm hex key - and US Sizes wont fit it neither.
Only way I know to turn those is to use a 4mm hexkey and a sliver or two of shim material, verrrry cautiously.
Or...get a 5mm and hone it down to 4.3mm and use a hammer on the damned thing, which is not exactly the reccommended way to do it - but it does tend to get the damn things out so they can be replaced with fasteners made by SANE people.
Hexhead bolts are EVIL, spawn of the abyss.