Well, I wouldn't expect the 1/2 divider to appear with DDR4-4000, but with DDR4-5000.
Anyways, what's the point to support so high DDR4-5000 clocks if the overall performance will be lower than with DDR4-2500?
If you have a problem, if no one else can help, and if you can find them, maybe you can buy DDR4-5000.

Seriously though how much does DDR4-5000 even cost, i couldn't (30sec checking) find any and even if you did have the money to buy some as DragonQ pointed out we don't know how high IF or the mem controller will be able to clock with Zen2, if Zen(+) is anything to go by it's around 1600Mhz but what exactly was holding it back from clocking higher is anyone's guess.