Here's a completely made up point of view. (I haven't read the articles.)

GPT-4 is not an LLM, but a complex software system, which has LLM(s) at its core, but also other components like RAG, toxicity filter, apologizing mechanism, expert systems, etc. "GPT-4" is product name / marketing name. For OpenAI, this would be logical for performance and business reasons. This explains also how they can tune it, the apparent secrecy about the architecture, etc.

It's also logical to make small, incremental changes to this system instead of building whatever GPT-5 would mean from ground up. So I expect "GPT-5" is also just a marketing name for a slightly better black-box (for us) system and product line.