PS5 is very likely going to have the best selling console launch in gaming history (this is, if it sells more than PS4). This means that considering they sold out preorders in minutes, shipments and storage are going to be massive, so probably had to reduce shipments for October and November to make space for all the PS5s they need.
In addition to this, maybe it also means some people won't buy a PS4 this month because they will buy a PS5 next month instead, and we also should remember there's a pandemic out there so many people don't go to the retail stores.
PS home consoles sold like a 3rd of its units after the release of their successor. PS4 on its 8th fiscal year is selling at record levels worldwide for that point of its life cycle, even better than PS2, the best selling ever console. And Sony announced they will support it during at least 2 or 3 years. So very likely PS4 will reach its 11-12th FY as PS2 did.
I mean, demand for PS4 this year has been there until September at record levels worldwide. So it didn't magically dissapear, the main reason must have been they decreased the PS4 shipments during October to prepare the insane PS5 shipments and storage for a record launch the early next month, November because probably they couldn't handle having both. And once they solve the PS5 supply issues, they will ship PS4 as they need.