I imagine it's the same foundation model on the 4 series, with Fable 5/Mythos being a new or upgraded foundation model. Then the point releases are fine-tuning plus post-training alignment with desired outcomes. The "thinking" can involve multiple steps, eg. asking the model first what it thinks the user wants to do, why it wants to do it, rewriting the prompt to generate better outcomes, how it should do it, come up with a plan, etc. So when they announce each point release like Opus 4.8, they're probably adding new layers of thinking to try and get good results on benchmarks. And that of course has cost and speed implications.
Then Sonnet/Haiku are just attempts to quantise/distil down to an acceptable performance/cost ratio. The cynic in me says we probably won't see any more of those until post-IPO, keep people addicted to the most costly models to pump a quarter or two of revenue figures, unless a competitor starts seriously undercutting them on price/performance. Hence the recent requests to slow down model training worldwide with their competitors.
Of course it could be that Fable "5" is just a marketing bump to the version, not a new foundation model...