Just checked with the Bosnian manufacturer. Looks pretty ok.
I didn't ask for 4 layer board but it looks like it's fairly proportional as other manufacturers are. So here is the example:
60 EUR Fee for a project start +
60 EUR per panel of size 305x460mm where 20mm padding is for each side.
I think that some significant number of panels can be more affordable. Also, they do all the paneling or what ever else you might need.
Also, another standard size of a panel is 530x410mm, price is slightly higher.
They don't do less than one panel.
Contact them if you need more info:
https://pcb.ba/Home/PCBp.s. They don't play with copper thickness, what you agree you get!