"I assume that a 4-4 teaching load at a school where the mean class size is 20 is pretty different from a 4-4 at a school where the mean class size is 100. Is this true? Can anyone speak to this?"
Yes there is a huge difference. With more students (but no teaching assistants), you get more questions and more individual issues to attend to. You're also limited in your pedagogical approach. In the classroom you may need to do more lecturing and less activities (though you may also be able to do more interesting things with clickers/demos the larger your class size—for this reason I kind of love the one 100 person class I taught). In your assessments you will be forced to do more multiple-choice and less written work on tests; less written work overall; fewer assignments overall.
Now on the flip side, if you LIKE to lecture and have multiple-choice tests and whatnot, you might get slammed by teaching evals and general expectations from the rest of the faculty when you're consistently teaching small (<20) classes. If this is you, maybe the 100-person course place is better. When students attend schools with small classes, they expect a certain level of pedagogy (not just lecture) and a certain level of interaction with you outside of class. But if you're teaching 4 20-person classes you won't have more than 80 students in a semester, and that is not a bad number (i.e. less than the total students in the hypothetical 100 student lecture.)
"And what does this mean for faculty workload: Is a 4-4 at a very small school going to be an insane workload or a reasonable one?"
I think that a 4-4 load is always a lot unless you're really not doing any research or much service during the school year (but I'd assume you'd be seeing advisees.) If you can have research with students "count" as a course, then you can be down to a 3-3 and it's so much more reasonable.
As for a small school 4-4 vs. a large school 4-4, it depends. More students per class = harder (generally), as I stated above. However, if you control for class size, it might be easier to teach a 4-4 load at a large school than at a small school. At a large school you'd be more likely to have 2 preps for your 4 classes (e.g. two sections of each course.) At a small school of under 1000 students, it is unlikely you'd get to double up on any of your 4 classes, and you'd usually be left with 4 unique things to teach. That depends on how the psychology major is structured, though. For courses like intro, stats and research methods you are more likely to be able to double up. At our school we also require students to go through a sequence of some content courses, so I can double up in my content area because I happen to teach one of these required courses. But in my other semester I always need to teach 3 unique classes (electives or things like intro, methods, stats).