Is this because you wouldn't be able to actually sit through a domestic flight if you were seated one row back? Presumably it would mean you would be waiting up to 8 seconds longer to disembark - this will really impact your life.
I love AFF, but the absurd amount of time and energy members expel on obtaining the first few rows of economy makes me laugh - unless its the very front row each row after that is exactly the same - you may just disembark a few seconds later. I really dont see how your life will improve by sitting in row 4 rather than row 5.
Row 4 on a 738 has more space/leg room/pitch (even if the J pax in row 3 reclines), more space under the seat in front of you for your bag and (some people may like this) tray table and IFE screen in the arm rest. So it's more than just who gets to leave first.
When I'm in row 5/6/7 I usually walk past the forward Y and J pax when I've disembarked and am walking through the terminal. Maybe I'm in too much of a rush.
Back on topic, I've only had someone ask me once (in AA F) - I was in first row aisle on a 737 (3D is it?), boarded late and a lady (50s+ ?) was in my seat. She wanted to be next to her husband and asked if I wouldn't mind swapping (she was in the second row window same side - 4E? I don't remember). I said OK but then in the middle of the flight had to get up, disturb the guy next to me and waste extra time to get to my bag in the overhead locker. FWP I know, but my mistake is that I took ownership of someone else's problem. Lesson learnt. Call me extreme, but it is what it is.