Not entering ISR for timer 4 in stm32f4
You not only have to configure the timer mode, interval etc. and enable it but you also have to tell the timer on what events should the interrupt be generated.
You can do this with a function:

void TIM_ITConfig(TIM_TypeDef* TIMx, uint16_t TIM_IT, FunctionalState NewState);

This is obviously because the timer may generate an interrupt signal for many different reasons, not just when it for example overflows.