The difference between a 4020 ROPS and a 4240 ROPS is the isolator (pretty sure it is only needed with a canopy), the strap/spacer to go between the axle and the ROPS, and longer bolts to accommodate the strap/spacer.  If you look at a 4020 hi-crop ROPS the spacers and longer bolts are included along with an extra plate and bolts to deal with hi-crop axle; that kit is $1922.71

FWIW I have a foldable ROPS I will be removing from a 3020 if you are interested, you would need longer bolts and spacers.  Make me an offer.