On opening day, the price difference is minimal (like $50 on a 7 day trip), but that can spread over time (more that $300). Our first was a 4A on the Fantasy, but our second was a 4B. Almost no difference. The only thing I'd say is on the first cruise, I was more likely to take the stairs up two flights to Cabanas and pool area, whereas on the second, that was 3 flights, so I took the elevator.
Really, just figure where you're most comfortable and what you think you'll be doing on the ship most and decide whether it's worth the extra for 4A over B.