London - 4 day itinerary

My rule of thumb is to schedule two major sights in one day, or one major and two others; anything else I manage to fit in is a bonus.

The hop on hop on is a good idea as it saves time and energy.
No problem with the itinerary but is possible since people have different levels of energy
that you will not do everything.

Forgive me if this is impertinent, but If your beliefs align, you might consider going to Westminster Abbey for your church services. When I studied in London for my Masters, my friend and I went for Easter. I had not even thought about it until she suggested it, so this is why I mention it. It was lovely.

I would add, if you are not able to do this, then I think you could do your river cruise, Westminster Abbey AND National Portrait Gallery/Trafalgar Square in one day. Imperial War Museum is further afield, so less likely to be done together. Though, if you are interested in a war museum, then I'd suggest changing it to Churchill's War Rooms, which is in Whitehall, so right down from Trafalgar Square and much easier. You could do all of that in one day without being too rushed, in my opinion, because you're so close together. I'd leave the river cruise till the end because it will be a nice rest as well. We did this, and it worked out nicely. The NPG has a lovely cafe at the top with great views of Trafalgar Square as well.