I hose out under the bonnet on my D4 regularly - have never had a problem.
Probably wouldn't recommend using a high pressure gun or similar under there, and don't direct a jet of water toward electrical components - but other than using common sense, I don't see any reason at all why you can't hose out under a D4 bonnet.
As to original post - have also hosed out under the bonnet of D2 Td5 on many occasions - have even (carefully) used a high pressure gerni-gun under there to de-grease the engine. Never had a problem.

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