Are some people more prone to plaque build-up on their teeth?

Actually it sounds like you have excellent homecare considering you haven't had a professional cleaning in 4 years. Some dentists go a little overboard with their patients regarding proper homecare and it sounds like this dentist is one of them. Relax, you are doing a fine job and all you need to do now is to schedule regular visits to keep things that way. A deep cleaning is only needed when indicated (periodontal pockets, radiographic signs of subgingival calculus, etc) and I seriously doubt that you need it done. But YES some people are more prone to develop calculus buildup than others and most of this is due to genetics. By the same token genetics also is the reason why some people who do not lift a toothbrush never seem to have any periodontal or teeth problems. Very few patients have perfect homecare and that is why 6 month or annual checkups are needed.