I’ve been using Unreal Engine for a long time, and I know the basics pretty well, but I want to improve my skills to potentially get a job in game making. What resource do I use to learn advanced game making with UE4?
That’s really vague, what part of game development is your focus? Do you have examples of your current work?
All depends on what you want to do - large teams tend to require more specificity within disciplines (art, programming, audio, etc).
How to make my skills job worthy?
Make impressive prototypes, videos, tech demos, or art.
I’m interested in something that requires working with Unreal, like game-play, or something like that (I’m 15, and I want to start learning those things so that I can be really good at it when I can get a job in that field). I’ve watched a lot of tutorials on YouTube and the Unreal website, but I want to learn what I need to learn to eventual get a job.
Every tutorial you watch should be able to help you extend your knowledge on methods of completing certain tasks. I suggest you really take some time to focus on a tutorial series with networking and really understand how everything works so you can replicate it with various different projects.
These skills you gain from the tutorials you watch should be sufficient enough to land a junior position at a company. Showing these skills in a portfolio on a website would be the easiest way to allow someone to view your work. You’ll learn a lot more as a junior developer than watching anything on YouTube or probably even in school.
You will learn best when you do projects. Just sit a make a small game I guarantee you will stumble on problems that you learn a lot from. 
Just keep pushing yourself. Always remain your own biggest critic and try to imrpove your work constantly. Never rest for too long on what you have achieved so far.
If you encounter problems, really the best thing is to solve them yourself. Thats how you exercise problem solving, complex thinking, flexibility, ingenuity and such. The skills that no one else can teach you but are at least as important as the actual technical and artistic experience… which you will improve along automatically.
I focused heavily in blueprints for about 1 year before I got lucky enough to score a position at my first company, but I had a decent background in devops and security from college.
I’d say focus on Algorithms and Math(Physics). If you want to focus onUE4, I’d build prototypes every month and do self-imposed game jams as well as larger official game jams. Get something out on steam and look a bit into networking in UE4.
I’d also say to try to make content that you can reuse for multiple projects. Thats fast and easy to use. Gameplay Programmers might be asked to build tools to help speed up development over multiple projects so knowing how to build great tools will help you be a great programmer.