(Disclaimer: I'm the founder of OpenPipe, one of the fine-tuning services OP tried and ultimately the one that produced the highest performing model, it appears.)
Data extraction is a use case that fine-tuned models are fantastic at, so I'm not surprised that OP got good results. That said, I've also found it's pretty easy to beat GPT-4 across many task types if you have a way of getting strong training data. We published some research[1] a week ago where we found that across 4 example tasks spanning creative summarization, question answering, data extraction and classification a fine-tuned Llama 3 8B was able to outperform GPT-4 on 3 of them. The key was to create a repeatable way of generating high-quality training data, which is also addressed in the post.