I liked the espalier plum, but it was a lot of fuss to keep it in order. They 'want' to grow out, they don't really want to be 2D, if I make myself clear ?
There's also their roots to consider, especially at a boundary. You haven't said if it's a fence or a wall or open. I'm presuming it's a fence, because a wall shades as well as protects and you didn't mention. Thing about a fence is that it has two sides, and you'd need to prune both, and if the other side is the one that gets most light, that's the side the tree will try to grow to.
On the whole, best agin a wall or a solid fence. On an open fence you can get a much better crop because even though the tree is trained along it, both sides will get light, even if one does better than the other, but your wires/stakes have to take the weight of the branches to keep them trained straight. Once established they're fairly stable, but they'll always try to reach for the light and the bigger the tree, the more quickly it'll grow.
I think I preferred the fan shape rather than sideways espalier.
You might find it easier just to plant fruit trees that have been grafted onto very dwarf stock. There are some truly excellent varieties around nowadays, and with the internet for searches it's a lot easier to see what's available and ask questions of the Growers.
A 4m tree doesn't sound that high. Yesterday I picked a crate of apples of my wee tree...it's not quite 4m, and trust me on this, it's too high to pick easily unless you have, and can use (my shoulders, elbows and wrists ache today) one of the fruit picker poles with the net bag on the end.
If I ever grow another I'd aim for no more than 3m at the very most.
Sorry
@TeeDee I don't think I'm being of much help. I think this is maybe more of a question for folks like BR. I have a small garden really, it's just rather packed