I have four cats...I want to start brushing their teeth but I don't know if I'm supposed to buy a different toothbrush for each cat? Would it be unsanitary to use the same one for all four?
Personally I don't think it matters. You can use one toothbrush for all the cats or label toothrbushes for each cat. If one cat needs a different toothpaste from the rest, say because she / he only likes one particular brand or flavor that none of the other cats like or something, then use a separate toothbrush for that cat and clearly label it with decorative washi tape or a marker.