AMD won't do DDR5 on Zen 4 simply because DDR4 has way more market share and it would be easier for consumers to just use their existing DDR4 with a new Zen 4 processor. I don't think either AMD or Intel would go to DDR5 before 2021.
As stated gazillion times already, one of the big advantages of the chiplet architecture with the memory controller on the IO die is that they can sell the same CPU chiplet on two different sockets with different IO dies, allowing them to sell both DDR5 and DDR4 cpus in a single generation. I thing it would be downright weird if AMD doesn't try being a DDR5 early adopter. Anyone who thinks DDR5 isn't worth the premium can still buy the newest AMD CPUs, so they lose nothing by doing so.