Hmm This is a tough one. You will be SO far away, that if he really needs something, heaven forbid, like surgery, what could you do anyway, other than feel horrible for being so far away, and pay the bill. Do you have someone who will groom him and watch over him while you are gone? I think he deserves that, just my opinion. Free lease is perfect, I have my old guy on one. However, since you will be gone 4 years, you will not be around to deal with lease ending, finding new person, etc. I have a feeling the BO realizes all this and that she will be the one, ultimately, there with the horse for the 4 years.
On the otherhand, altho the 7 yo may not be perfect, with lessons stipulated in the agreement maybe?, perhaps she would be. If the parents are looking for a somewhat longterm commitment, but are afraid the kid will lose interest, perhaps the free lease for X years would be a great situation for all. You would have a longer more committed lease agreement, keep the ability to make the decisions, ultimately, and they would have a horse for the daughter, who, at 7 probably has a few years until she really finds boys and loses interest.
Definitely worth thought, and would take written agreement for sure.
Good luck. I free leased mine to a therapeutic riding center. He loves it, has lots of love to give and get, and I have the ultimate say. He will eventually come back to me if/when he can no longer help them, and live out the rest of his days as a pasture puff. I also would never have given up that control. He is part of the family.