Personally, I feel like neither GPT-4.5 or GPT-5.5 is very likely. In the case of GPT-3.5, it was a very idiosyncratic situation where they had a model that could generate any text, and then they modified that model to follow instructions, which significantly changed what it could do. On the other hand, GPT-4 already follows instructions. Maybe they could make a change to GPT-4 or GPT-5 that's comparable in size to the change they made to GPT-3 to get GPT-3.5, but it's far from assured that they'll do that instead of focusing on training another brand-new model.