If I was in this situation, I will offer row 4 to row 3 since row 3 is a 'better' seat plus there is a chance that they will have two options for thier meal choice as well as some kind of a small gift (i.e. box of chocolates, a calling card, etc.). They may decline the 'gift' but at least you offered. You are asking someone to move from an aisle seat to a window seat on a 6-hr flight. It will be a different story if the person sitting in the window seat could get up without asking the aisle person to get up...I remember flying FC on CO years ago where you could get up from a window seat without asking the aisle person to get up and etc.

It is my recommendation to ask at the gate. I was traveling with my wife and son (we were in FC) and I was called to the gate at PHX to exchange seats but I declined since the switch will require my 3-YO son to sit by himself.

Another option is for your son (4C) and daughter (4A) to sit in 4A & 4C...with one of you sitting in 3A (so if your daughter kick the seat it won't bother another FC passenger) and the other one in 4F. The one in 4F could switch with your son (4C) from time to time.