1. N3B TSMC node used by M3 is reportedly low yielding. Apple likely wants to move away from it asap and move to N3E, which is the higher yielding node.
2. Rumors are that M3 Ultra is not planned due to low yields. M3 Max die shot does not have UltraFusion silicon. This supports the first theory.
3. Apple base the M chips on the A series cores. A18 Pro has already finished in design because Apple always starts making A series chips in Spring before a Fall debut. Therefore, A18 Pro is already in production using N3E node. Given enough early planning, it isn't surprising that M4 can be released early.
4. Reportedly, Apple plans to focus on GenAI at WWDC. They're also behind in GenAI. In order to catch up, they need to be aggressive. M4 is likely to have a big improvement in inference speed via the NPU and/or GPU.