Whilst DL and BA have an interline agreement, I do not believe that in your case you can check-through. DL will not accept your bags in GVA. It's not that you don't have a layover technically, you don't have a layover, you have two separate flights. You're going to have to collect your bags in LHR and then make your way from T5 to T4. You won't be able to use the Flight Connections system so you'll have to take the Heathrow Express and train to get to T4.

Honestly, I think two hours is really cutting it a bit short, especially if there is any delay in your inbound from GVA. Not to scare you too much, but if you miss your DL flight to LAX, the return of that will automatically be cancelled. This is because you will be deemed a no-show for this flight (rather than a late arrival; you are not connecting). All a bit risky I think. I fly in and out of LHR several times a week, and for something like this I would have left at least three hours.