My experience with routes listed as 4th class is this: They have been routes where the actual physical climbing did not require any special technique(like say as hard as really steep stairs or a ladder, or getting onto a bunk bed), but a fall could be serious or fatal and a rope is highly recommended. Sometimes class 4 has just put me on exposed ledges I can pretty much walk on, maybe using my hands for security. If the rock is loose, the route description usually(not always) says something about that. On the other hand if the rock is solid, it is sometimes stated. If nothing is stated I pretty much prepare for whatever, but the rock quality hasn't been an issue on that route. Sometimes I've done a route listed as 4th class, but was actually, in my opinion, more like low class 5. Others may have different views and/or experiences.