You can actually breed for butterfat content as well. Some "families" have better butterfat's than others- we have a British Toggenburg who always has really good butterfats- and she is bred from Webster & Smith's BT line, which all have good fats. AN's naturally have the highest BF, but can give a lower yield, and sometimes more importantly, a shorter lactation, meaning they might need to kid more often in order to keep producing milk.
Anke is right- BF do vary through the lactation. We have had milkers who might have low BF in the summer during their first part of their lactation due to the volume of milk they are producing, but then by the following summer they are back producing 4.5%
Beth