Rumor: AMD is releasing Zen 4 CPUs with DDR4 support for AM4 motherboards
AMD may be working on a separate series of Zen 4 CPUs that will be compatible with existing AM4 motherboards. This should allow the chips to support DDR4 memory, as the AM4 socket only supports that generation of ram.
For the time being, it seemed that AMD would switch completely to DDR5 memory with the upcoming processor architecture; there would be no DDR4 controller I/O die in a Zen 4 CPU. According to the usually well-informed leaker Greymon55 on the other hand, cpu resellers have learned about the existence of AM4 compatible Zen 4 chipsets. These are product plans, so whether such chips will indeed be marketed is not yet certain.
According to AMD, the first upcoming Zen 4 CPUs in the Ryzen 7000 series will all be made for the new AM5 socket for the time being. AM5 emphatically does not support DDR4 memory. The company has therefore hinted several times that this generation of ram will also be supported to a greater or lesser extent and that AM4 will continue to exist alongside AM5. But whether this applies to brand new chips is not clear.
For the time being, it looks like PC builders will need a new motherboard and new ram sticks to use a Ryzen 7000 CPU. With a special AM4 variant of these or not yet announced Zen 4 CPUs, AMD could potentially appeal to a large otherwise skipped target group.