PIC's ports individual bits manipulation - how to?

[flotulopex]

Hello,

In one of my projects, I have to adress the first (LSB) four bits of my PIC's PORTA.

This is done within a FOR...NEXT loop so the bits are adressed as a variable ("Digit").

I would like to use the other four bits for other purpose (Button, any Input...) but I can't make this the way I did it up to now.

How is it possible to handle ONLY those four bits wihtout modifying the other (MSB) four bits?

I've attached my code if you like to see how I've done it. Any comments/critics are very welcome.