AMD may be working on a separate line of Zen 4 CPUs that will be compatible with existing AM4 motherboards. Because of this, the chips should support DDR4 memory, since 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 CPU resellers, on the other hand, have learned of the existence of AM4-compatible Zen 4 chipsets. These are product plans, so it is not yet certain whether such chips will indeed be marketed.
According to AMD, the first upcoming Zen 4 CPUs in the Ryzen 7000 series are all made for the new AM5 socket for the time being. AM5 emphatically does not support DDR4 memory. The company has therefore already hinted several times that this generation of RAM will also continue to be supported to a greater or lesser extent and that AM4 will continue to exist alongside AM5. But it is not clear whether this applies to brand new chips.
For now, it looks like PC builders will need a new motherboard and new ramsticks to use a Ryzen 7000 CPU. With a special AM4 variant of these or as yet unannounced Zen 4 CPUs, AMD could potentially appeal to a large otherwise skipped target audience.