di2 is totally unnecessary. Discs make sense in terms of future proofing - though I would still be reluctant to go that way until things are more standardised.
It's a tricky time to spend a lot of money on a bike in that respect.
I usually agree, but if someone is happy to spend 4k...
Actually can easily get a custom frame with full spec for that money, which bring my comment to simply buying a 1.5k carbon road bike to get into cycling first.
If £4,000, get Di2 hydraulic disc.