If your son had his 4th birthday before 1st Sept 2014, then he'd be in the current reception year i.e. his future classmates are already in school. I think that would mean an "in-year" application i.e. you would be looking for places out of the normal admission process as you would if you were moving school later.
You can apply and get a place from abroad (we did) but you don't get your first choice school, or anything that anyone else wants. After children in care, special needs and siblings, places are allocated according to distance... and you are/ we were the furthest away by a factor of 500!
This was a nightmare for us when moving back. Eventually we got a school that seemed ok, but wasn't when we got to know it. So, we had to move again a couple of years later... and this time we only got in to a better school by the skin of our teeth.
Its all much worse for the first three years of primary school (viz. Reception, Yr1 & yr2) because there is a cap of 30 on the number of children who can be in a class. From year 3 onwards, they can vary it by adding 2, 3 or even 4 more children at the school's discretion.
I'd look at finding a school, pref with a backup option and finding a place to put down as your UK address. Could you theoretically move back early with your children, say, to your parent's address, leaving your husband to follow? Then you have the address, and if its too late for your 4year old, its not too late for the 3 year old. So they could get a place at your preferred school and your 4 year old would joining the waiting list for his year, either at or near the top because of the sibling policy. then you just have to wait for someone to leave...