The problem with R4 GTL is that it is a year-related number dating from 1997-8 : which is newer than any R4. It is not legal in the UK to affix a number plate which implies that the car is newer than it actually is. So R4 GTL could never be put on an R4.
The following plates would be OK, if one could afford them : R 4, REN 4, 4 L, 4 TL, 4 GTL, A4 CAR, etc . . .