In my case, I could not downgrade to a 3.x version of integration services because my existing solutions did not support it. The key is to install the "BI Developer Extensions" (formerly BIDS Helper)". After installing it I was then able to see the biml commands in right click menus (such as "add new biml file", "expand biml file").

Confirmed this works with:

Visual Studio 2019 pro - 16.11.50

SQL Server Integration Services Projects - 4.6

BimlExpress - 2019 R1