I  burned most of a day figuring out how to make TIM4 function on an STM32F4Discovery board.  I'm a novice at programming at bare hardware level, but finding the information I needed was maddening. "Selects internal clock", which clock?  There are lots of them!  There seems to be good support from STM for C programming in the form of libraries that hide the complexities, but at least for now I'm committed to using Mecrisp.  Which in turn implies I'm coding peripheral operations in assembly.  Scope traces and code in a few days.  I've still got PWM output tol do.  But hopefully I learned something applicable today.