class 2 = an easy scramble. you need to use your hands a bit, but not consistently
class 3 = you are using your hands constantly, but the scrambling is not difficult
class 4 = a difficult scramble where less experienced people will want a rope for mental security even though they can do all the moves no problem. overlaps with low 5th class. practical difference is mostly related to other factors than how hard it is, namely exposure, greater position on a route or whatever.
Class 4 can include both simul climbing and pitches which are belayed but on which pro is not placed (leader solos dragging rope for benefit of second) as well as pitches which are 3rd classed but which the scrambler feels are more difficult than class 3 and should be noted as such.
For instance:
Class 2 - south ridge of tricouni Peak from meadows to west; summit ridge of McGuire
Class 2-3 - chimney on Black Tusk; west ridge of Needle Peak; Crown Mountain summit pyramid; SE slopes Mt Truax; standard route on Lady
Class 3: south ridge of Welch; SW slopes oF williams Pk.; w ridge N twin sister if you deke out right on the face wherever things get tough
Class 3-4: east ridge of Welch Peak; west lion Tourist Route
Class 4: Castle towers central summit (crossing notch); NE ridge needle pEak; W ridge of N Twin sister following crest; N ridge of Shadowfax; standard route on Rexford; north ridge of Tricouni