My suggestion would be to make a CS:GO map. This will teach you better than UT4 since player contact points and firing angles have major impacts on map dynamics. A small hole in UT4 might not ever get used, but in CS every millimeter counts. You will see how small alterations can dramatically change gameplay. That is pure level design though, chances are you’ll be working on games that have bigger and more detailed levels than a CS:GO map. So after you’re done with that, UT4 is indeed a great spot. That is if you want to get used to UE4. In all honesty though a Fallout 4 mod would do you better, since they have released the needed SDK for mod creation. My suggestion would be to make a small side-quest type mod. You could contain the whole adventure to a cave or a building, and your goal would be to get as much player engagement out of it as possible. Detail, verticality, hidden areas, lighting, enemy spawns, multiple entry points, cover and mission objective location are crucial for that. Story elements should blend well with the environment, things like pictures, logs, visual cues to past events. Whatever story you choose, it is advisable to have half of it implied just by the level. A simple example would be something like a skeleton with a dagger stuck in the ribs, pointing to a wall. The player might assume that a struggle happened, a close friend to the deceased has betrayed and killed the victim. During the persons final moments he attempted to escape/secure something, and at that revelation the player might figure out that the wall is fake and there’s a hidden lab or something behind it. You get the idea.
After you are done with that, your final step to tackle would be a more open level, such as a town, a forest or a big ship. Something that has multiple rooms, both open and enclosed. That should be done in your engine of choice (I’d choose UE4 any day, but it’s your show). The open parts of your level should have as much thought behind them as a CS:GO map, your enclosed areas as much detail as your Fallout 4 map, and the overall level as much story peppered throughout as your mod quest. That’s all I got.
Edit: Also try to learn the art of stealing. Reuse assets in places people won’t notice. No need to ask your art team to model 15 benches and 10 chairs just so that you can avoid the player noticing the same mesh.