For some, it might be enough to strangle a puppy. And as you progressed in evil, murder most foul might not be sufficient to tear at your soul. When you've killed four, it's easy to make it five.

You would think so, but that doesn't seem to be how it works in canon. The diary and Nagini were both Horcruxed with one murder. In fact, it's suggested that making Horcruxes makes your soul "unstable", making it easier to make more (canon HP was even unintentionally pseudo-Horcruxed).