There is no gpt5, and gpt4 gets constant updates, so it’s a bit of a misnomer at this point in its lifespan.

It’s possible to apply a layer of fine-tuning “on top” of the base pretrained model. I’m sure OpenAI has been doing that a lot, and including ever more “don’t run through puddles and splash pedestrians” restrictions that are making it harder and harder for the model to think.