Resigning after 4 weeks turned out to be much easier than I imagined
The whole thing turned out much better than I had built it up to be in my head. I woke up early yesterday and talked to my manager before the shift. I just told him that I was grateful they had chosen me to work with them, but that I had received another offer that was more suitable for where I want to get to in my career. He didn't make it awkward or anything; he basically said, "Okay, no problem, you'll need to give two weeks' notice," and that was that.
Honestly, I had expected a lot more tension, so that was the first thing that really relieved me.
When I got to work, I told one of my coworkers before anyone else. He smiled right away, congratulated me, and shook my hand. After that, I let the rest of the team know, and they were all very supportive. No one acted weird or upset. One person even said that this was probably a good step, leaving while I still could.
A few of them also said they could be references for me when I asked. They're good people, and that makes it a bit bittersweet that I won't be staying with them longer.
So yeah, the part I was nervous about is over now. Ready for whatever comes next.