Resolved! STM32F072 Clock tree section shows how to select clock for USARTS 1-3, but not USART4. Were in the reference manual (RM0091 Rev 10) does it show how to select the clock source for USART4?
The USART section states that all 4 USARTs can be clocked from a common set of 4 clock sources, but doesn't reference how to specify the clock for USART4 either.