Hi,

The most annoying thing in your project is ... the PIC must drive continuously the 4 digits AND while that, must measure the rotation period.

I think you'll quickly understand you'll have to use interrupts and ... a 16Bits timer ... what the 16F84 doesn't have aboard ...
so, you'll also have to watch for TMR0 overflows ...to increment a 8 bits counter for the timer Upper 8 bits counting.

Not really easy ( "asm" recommended here )... but doable.

seeing your pain for the first steps ... I'd recommend you to switch to a 16F628a that has everything to EASILY realise such a RPM meter. ( I mean the capture module ... )

The project is yours ... you are the one who decides !!!

Alain