It depends on what your build tooling looks like. If you are not using build tools and just using the CDN, then, yes, you can use Dojo dijits and other tools with 4x. It's not recommended, as Dojo 1 is simply in maintenance mode with varying bug fixes. But if you need to transition over a few cycles, then you can. You should move your custom widgets to some other framework tooling or web components.
If you are doing Dojo builds of your application, you can, but again, it is not recommended for 4x and is considered a deprecated practice. We have a repo here you can use for guidance, but no guarantees that it will work 100%. https://github.com/Esri/jsapi-resources/tree/master/4.x/amd