There are other thread on this, but there are two factors at play here. If purchasing a seat it is cheapest to do it at the time of booking, the price will rise thereafter. If you then decide to change your seat then you map get lumped with the difference in fees. Sometimes the fee goes down, but at least on paper not below the price before ticketing. That's Dynamic Seat Pricing, there are various threads on it in the forum. The other aspect is that BA does from time to time increase seat fees, with similar results. So if you bought after ticketing, the seat fee can change without Dynamic Seat Pricing coming into it, and again you could be done for the delta if you change. If the change has an involuntary component, due to equipment change, I believe you can ring up and they will waive fee differences if you have a good case.

4A is a good seat however the window alignment isn't so good.