Am I limited by the pKa value of an weak acid when preparing a buffer solution of it (and its salt)? If I was to make an acidic buffer system with the ph of, for instance, 4, would I be limited with my choice of weak acids? That is, how big does the difference between pH and pKa have to be in order for the weak acid not to be usable? Or is that not the case? I am very well aware that the optimal weak acid for has to have a pKa value quite close to the pH of the wanted buffer because they offer the most buffering capacity.
Also - Can someone hook me up with some equations applicable for making/dealing with buffers other than the Henderson Hasselbach equation? Are there any equations that can help me predict the buffer capacity?
If I was to make an acidic buffer system with the ph of, for instance, 4, would I be limited with my choice of weak acids?


Yes you are limited to weak acids that have a pKa of +/-1 of your desired buffers pH value. This is a rule of thumb used in many labs.Some theroetical background is quite nicely explained on wikipedia wikipedia#buffering  capcacity.

This reference also includes some useful equations related to buffering capacity.