I too was looking for the widgets in the TT4 theme and was frustrated to find they were missing while there’s no clear guidance on a replacement. As jordesign suggested, you can manually add blocks anywhere you want, but this does not replace widgets where you can update the content in one place and it will update all instances. This is the real benefit of widgets.
You can add blocks to the footer or header, but if you don’t want the same blocks on each page this will not work. Also, as far as I can tell, there’s no native way to add conditional logic to a block to determine when and where it might be shown.
Another option is to use template parts which are in some themes like TT4. This has no conditional logic but provides a reusable part to add to pages.
If you need conditional widgets/blocks like I do, for varied content in a section on multiple pages/sections that can be updated in one place, I think there may be some option with JetPack or a plugin like conditional blocks.