Your options are now effectively 4o and o3 (GPT-5 and GPT-5 Thinking). There's no equivalent to o4-mini (at least not via ChatGPT -- I think you could emulate it through some combination of gpt-5 model size (-mini?) and reasoning effort).

o4-mini was the best reasoning model in my experience, because it gave you approximately o3 performance in a very small fraction of the time o3 would take.

ChatGPT's GPT-5 Thinking is even worse than o3, it's closer to o3 Pro! Some messages take 4 minutes to get a reply. Which is great when that's what you want, but the niche of "5 seconds to get an actually intelligent response" has been deleted for no good reason.