Think of 4.5 as being the lacklustre major upgrade to a software package, pick one maybe Photoshop or whatever. The 4.0 version is still available and most people are continuing to use it, then suddenly 4.0 gets a small upgrade which makes it considerably better and the vendor starts talking about how the real future is in 5.0.

I wish OpenAI had invented this but it’s not that uncommon.