Nice choice to have to make (although I don't 'do' diesels), please keep us updated with pics of, and a write up on, your final choice.
Hi all If I may add my 2 cents worth... I’ve owned a few 3 series coupes (which the 4 series replaced) and have driven the 4 series too.(purely as a loaner) I currently have a 6series gran coupe petrol. Specced with a number of options.. sunroof.. b&o sounds.. soft close doors to name a few. looking back... I really love the 6.. it feels solid.. luxurious.. and just a solid car. The 4 series I drove felt more of a sportier vehicle.. I found it quite low, but the 6 is totally opposite.. anyway.. my opinion is go with the 6.. (not meaning to offend any 4 series owners) All the best in your search