I still remember going to London for the first time from Blackpool (Kellogs Cornflakes kids go free) going on the Piccadilly Line and seeing the word "Cockfosters" I was 10. I was fascinated by it all, and still smile today at 57 years old.
To fly at 4, the fact he is doing it with Dad will be a memory, that, although it will become more distant with age will remain
He will just love being with you and sharing any experience so point out stuff you like, for that is what he will remember, just as the Imperial War Museum remains close to my heart as my dear Dad Z'L shared it with me.
Have fun, don't try too hard, enjoy.