I'm confused with Bits. I know they are arbitrary as far as length goes which leads to my confusion.

I'm reading Applied Cryptography (Schneier), and i'm confused as to the size of such things, well not bits i guess but like bytes maybe. I know a char or int on the computer is 8 bits right? Like how would i implement something if i needed, suppose a 4 bit variable?

And how can i manipulate the bits on a bit-by-bit basis?

Thanks!