How can full-time lecturers find the time to prepare for lectures?
Two points:


Preparing for a lower division course takes much less time than preparing than preparing for a graduate level course for several reasons.  First, you presumably have much better knowledge of the material in the lower level course.  Second, you tend to present material at a slower rate in a lower level course.  Third, you should be using more active learning strategies (having the students do work in class) in a lower level course.  I've found that although it takes an hour or two to prepare for an hour long class in one of my graduate courses, when I've needed to substitute in a lower division course (e.g. calculus) for an instructor who is out sick, I can typically prepare for the class in about a half hour.  
it's much easier to teach a class if you have taught the course in previous semesters, and its particularly easy if you're teaching multiple sections of the same course in a semester.  So, an instructor with a four course teaching load might actually be teaching three sections of one course and one section of a second course.  This is described as "four sections, with two preps."