To get the ability to hire the fourth Engineer, you first need to rank up to level 21. Under 'My Team' (unlocked when you buy your first car) -> 'Facilities' -> Accommodation, then buy the grade A Accommodation (costs $100K). Finally, open 'My Team' -> 'Staff', and fill all four Engineer positions
(Grade A engineers will now be available).
The grade A accommodation is also the second grade A facility you will become available, when you've purchased three you'll achieve:
The Tooled Up achievement in DiRT 4 worth 33 pointsUpgrade three Facilities to Grade A quality