1 sq yd is 9 square feet so an area about 7 feet x 10 feet or maybe 4 feet x 18 feet. Once they are fully integrated it could be big enough but the area being small like that integration can be rough. Polish are typically bantam and these four chicks appear to be full sized, so there probably isn't any real difference in size. That eliminates the possibility of a safe haven type integration where the young ones can get through a hole the older ones cannot. Not sure what you have for a coop, if anything. In Jerusalem they won't need much in your winters. Summer heat is more of a danger.
The way I understand it the chicks are in that 1-1/2 square yard area across wire from the older ones, hens for sure an probably the rooster. I'd leave them like that for a week before I tried to merge them. Be around when you try so you can observe them. The area is small so it may not go well but you might get lucky. You may need to separate them and try again later. Base what you do on what you see.
I'd keep food and water in each end. Some clutter can help, especially in a small space. By clutter I mean something they can hide under, behind, or over to break line of sight. A typical example is leaning a wooden pallet against a wall they they can get behind. Fasten it at the top so the wind cannot blow it over and crush them. Some people put a table in there that they can get up on. A satellite dish might give them something to hide under, maybe up on blocks. You need to be able to move in there too. Small areas can be challenging.
Your area is fairly small, that makes it harder. But with patience you should be able to manage. Good luck!