Anyone who has even considered looking at getting a Pi knows that the last few years have been a challenging time. There's been such a shortage of Raspberry Pi boards, that scalpers have been selling on some sites for 3x their value.
If you've looked recently for a Pi 5, you might have noticed they are often available from a multitude of distributors. It's not that the demand has gone down, it's that the Raspberry Pi Foundation is churning them out at astonishing speeds.
Currently there are 70,000 Raspberry Pi 5 boards being produced every week, and the Raspberry Pi CEO Eben Upton has confimed that they will be scaling up to 90,000 in the near future.
As the Pi 5 isn't a drop-in replacement for the 4, there will still be a need for be old model to be produced as certain cases and hats won't fit the new model. As such they are also still producing the Pi 4.
As the Pi 5 isn't much extra in financial terms, most buyers will be looking at buying the Pi 5, but it's good to know that the Pi 4 will still be available and that manufactuing can switch over to the 4 if needed.