Turris OS 4.0 switched switch and VLAN configuration from swconfig to dsa due to changes in upstream OpenWRT and Linux kernel. VLANs can now be configured in the interface configuration tab in LuCi.
E.g. if you want an interface to be present on Lan 4 port with vid 10 you have to add the Virtual Network Interface Lan4.10 to the interface‘s bridge. This will not create a software bridge, but will set up the switch chip using dsa.
See explanation from cynerd here: Turris OS 4.0 beta1 is released!
He also provided a link with background explanation of the dsa system. DSA was created to configure switch chips using standard linux tools and to eliminate the need of a tool like swconfig that always required users to set up their switch manually AFTER they already set up their interfaces using standard linux tools.