Watermarks are still possible using Lightroom Classic, which still utilizes a local catalog and stores images locally like Lightroom 4 did. However, the version of Lightroom you have installed is the new version that stores images in the cloud, and the desktop version does not have a watermark feature at this time.
You can create a watermark with the mobile version of Lightroom, but not the desktop version. I have to assume that means watermarks will eventually make their way into the Lightroom on a computer.
But, since you were using Lightroom 4, you probably should be using Lightroom Classic now. It is the same application as Lightroom 4 with a few upgrades. Lightroom Classic has the watermark feature that you are familiar with from Lightroom 4.
Lightroom is missing a lot of features. It's still a relatively new application and over time it will get better. But there are other reasons to use Classic instead. Lightroom uploads your photos to a cloud. Classic does not. If you used Lightroom 4 before, Lightroom Classic will be essentially the same.