With modern high-quality datasets and the plummeting H100 rental costs it is 100% a feasible undertaking for an individual to train a model with performance
far closer to gpt-4-1106-preview than to gpt-2, in fact its difficult to train a model that performs as badly as gpt-2 without carefully selecting for datasets like OpenWebText with the explicit purpose of replicating runs of historical interest: modern datasets will do better than that by default.
GPT-4 is a 1.75 terraweight MoE (the rumor has it) and that's probably pushing it for an individual's discretionary budget unless they're very well off, but you don't need to match that exactly to learn how these things fundamentally work.
I think you underestimate how far the technology has come. torch.distributed works out of the box now, deepspeed and other strategies that are both data and model parallel are weekend projects to spin up on an 8xH100 SXM2 interconnected cluster that you can rent from Lambda Labs, HuggingFace has extreme quality curated datasets (the fineweb family I was alluding to from Karpathy's open stuff is stellar).
In just about any version of this you come to understand how tokenizers work (which makes a whole class of failure modes go from baffling to intuitive), how models behave and get evaled after pretraining, after instruct training / SFT rounds, how convergence does and doesn't happen, how tool use and other special tokens get used (and why they are abundant).
And no, doing all that doesn't make Opus 4 completely obvious in all aspects. But its about 1000x more effective as a learning technique than doing prompt engineer astrology. Opus 4 is still a bit mysterious if you don't work at a frontier lab, there's very interesting stuff going on there and I'm squarely speculating how some of that works if I make claims about it.
Models that look and act a lot like GPT-4 while having dramatically lower parameter counts are just completely understood in open source now. The more advanced ones require resources of a startup rather than an individual, but you don't need to eval the same as 1106 to take all the mystery out of how it works.
The "holy shit" models are like 3-4 generations old now.