Find something you’re interested in and work on it. Whether it’s a UE4 project, a plugin, or even the editor itself; open the source code for it and browse its source code. Now that you’ve learned fundamental concepts, you should learn how (and when) they’re put to use.
One thing to keep in mind as you develop your programming skills: each type of component (project, plugin, editor app) has a lifecycle that dictates where it fits into the software ecosystem and what it can do there. They all have ways in which they start up, interact with their environment, and are terminated (shut down). Knowing this helps you focus on the details you’re interested first, and ignore the details that don’t interest you at the moment.
For example, if you want to add a special ability to a game character, you might start from a project that does something similar and study how it was done (without trying to figure out all the details about how projects or plugins work). Then you might try to add the ability you want to another game project. This would allow you to focus on the details of character abilities. This strategy could be applied to other topics of interest in the UE4 system (or other software system in general).