Hi Pavel,
If you are building pgAdmin4 from the source, then you don't need to install pgAdmin4 from the wheel.
After taking the pgAdmin4 source from git HEAD, you need to install nodeJS, Yarn to build JS/CSS modules. the error you are getting is possibly due to this. For more help, you can follow the steps documented
here
It seems QtWekbit compilation went well. so just run pgAdmin4. it should work.
Also, can you share logs generated at location '/var/log/pgadmin4/pgadmin4.log' ?
Thanks,
Surinder