Betting NO. OpenAI is at GPT-5.4 as of March 5, iterating 5.x with roughly monthly cadence. The natural next increment would be 5.5, but three paths lead to NO: (1) they jump to GPT-6.0 branding for a generational leap, (2) they rebrand entirely (like the GPT-4 to o-series pivot), or (3) they skip 5.5 the way they skipped o2. OpenAI naming is genuinely unpredictable. I put this around 50%.

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.