So non-Cosmetic vehicle slots. How many of them can be on at once?

[LectraWyraph]

There is another square on the Vehicle slot thing on the side of the left slots not appearance in order from left to right of... Utility Defense and Performance, and then a fourth slot which is faded out I don't know if that's another defense or performance or what.

Can vehicles only be given three upgrade slots, or is that fourth square there for a reason? Can there be two performance slots or defensive slots?