The R2j and R4j are the same bike, there are "R (number) (letter)" codes for all the NSR250 variants, they're all on the front page of the parts manual for that model.

The R2j is the stock mk1 mc18, the R4j is the Rothmans (SP) model, the sole difference is the wheels.
Both use the seperate CDI and power valve control units.

The R5k is the stock mk2 mc18, the R6k is the silver Terra (SP) model, fancy suspension, wheels and a dry clutch are the differences.
Both use the PGMII integrated CDI/power valve control unit.
_________________
Please do not PM me technical questions, if you can't find it on the Forum start a thread