Hello Zolko,
I have tested your assembly and I must admit it is great, and you are on the right path, when we are talking about assembling. There are a few issues I have with assembling with your Assembly 4. These issues are maybe already resolved, but I was unable to find anything about them here in a FreeCAD forum. It is possible that I wasn't looking careful enough, and if that is the case, I apologize for bothering you.
I was trying to use Assembly 4 for assembling cabinet furniture. Like in the picture1, I have created one part named "Panel". Then I linked that part four times to create left, right, bottom and top side of a Model named Box. I have used Variables to set dimensions of those panels. So far everything worked as expected.

- picture1.png (23.7 KiB) Viewed 464 times
My next step was to assemble several Boxes in one ParentAssembly. My intention was to use Box assembly similar to the usage of the Panel part in the Box assembly. I simply link Box assembly several times to get a ParentAssembly with lets say four boxes. I managed to do that, but not like I was expecting. Box was linked, but Variables were empty. Like in the picture2. Everything was there except for Variables.

- picture2.png (15.48 KiB) Viewed 464 times
My questions are as following:
- Is it possible to "transfer" Variables with a link to an Assembly Model?
- Is it possible to "detach" link after linking, but to keep all the properties of a linked part? I would like to link my Box and then "detach" it and change Variables so my Box will have some specific dimensions. Then I would like to link my Box second time, but this time with different dimensions. That way, I will have two Boxes with all the properties of an original Box, but with different values.
Thanks in advance.