Hi,

If it is an existing board ... I'd first carefully clean all the "touched" PCB , verify all Pic pins are correctly soldered ( Laugh, ... who never forgot a pin ??? ) and no short ...

Magnifying glass required ...

Then, if no success ...Try you chip on a breadbord with the ol' famous "Blink" program ... you'll know if it is OK. and
where to look at ...

Alain

PS: 4 Mhz chips are not supposed, but work really fine up to ... 28 Mhz !!! ( no µchip warranty ... of course ...)