My recollection is that a D1 has a designed max towball weight of 150kg (or is it 190kg) - so this has got to be the limiting factor - will a 3.9t tow have a tow ball weight of more that 150/190kg - sure will if it is loaded correctly.
So even if the tow bar arrangement can take the 4 tonne, with the correct brakes and all other requirements are met - a D1 is still limited to only 150/190kg of downforce on the ball - the only way would be to modify the tow vehicle and have it engineered - I have no idea what be required.
Garry
REMLR 243
2007 Range Rover Sport TDV6
1977 FC 101
1976 Jaguar XJ12C
1973 Haflinger AP700
1971 Jaguar V12 E-Type Series 3 Roadster
1957 Series 1 88"
1957 Series 1 88" Station Wagon
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