The 4.0 is generally the "S" model while the 4.6 is the HSE and has "all the fruit" and somewhat more torque/power.
If you can find a good example post mid-1999 however, known as the "Bosch upgrade" -or Thor engine (VIN from XA410482 onwards) try and get that in preference: it gained Bosch motronic EFI and electrics, an upgraded gearbox (ZF4HP24), 4-pins diffs, improved WABCO Brake modulator, 150 AMP Bosch alternator,improved ignition coil setup, improved Crankshaft position sensor design/reliability, and sundry other changes...
cheers
EDIT: It's just an impression I have BUT: the Disco 2 with the V8 option (4.0 litre) is in many ways identical to the P38a in its running gear wrt engine and transmission... yet does not seem to attract the same criticism re. reliability... just an impression as I said .... which raises the issue as to whether the P38 is indeed inherently more unreliable as its reputation implies...or its relability is no worse! I'd welcome some informed comment!!
MY99 RR P38 HSE 4.6 (Thor) gone (to Tasmania)
2020 Subaru Impreza S ('SWMBO's Express' )
2023 Ineos Grenadier Trialmaster (diesel)
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