OK here you go, on the left is a standard 3.9 plug Champion RN11YCC and on the right is the Champion RC11PYPB4 that fits my 4.0 (which has the "Thor" engine/Bosch engine management/four coil dizzyless ignition system)

Everything else is smaller except the thread and reach which is identical so the good news is that they will physically fit

According to the "spark plug bible" at work, the rest of the plug number is:
R - resistor
C - the physical dimensions 16mm head instead of 21mm; reach is 19mm and thread is 14mm same as the RN ones
11 - heat range
PYPB - Platinum tipped electrodes with insulator projecting 1.5mm and tapered off ground electrode (same projection as the YCC except platinum)
4 - plug gap 1mm which I make 39 thou in the old money
It says somewhere else that you mustn't adjust the plug gaps on these as jamming a feeler gauge in there will damage the platinum coating, and since that bit is what presumably costs £7 odd I guess you wouldn't want to do that