I was gonna say it depends on your relationship to your inner mountain goat... but actually I like Rosebrough's succinct, rope-based description in The San Juan Mountains (1986, pg 9), perhaps because I've been using it for a long time:
3. Unroped climbing requiring the use of handholds.
4. Roped climbing where a leader climbs without protection but gives the other climbers a belay.
He goes on to give a paragraph of elaboration on each class, using examples in the SJs as calibration, and saying 4th Class is "the most subjective". With a continuum of varieties of terrain and qualities of rock, trying to place all climbs into a small number of discrete classes is obviously going to create borderline instances.
The key word there IME is leader. A 4th Class route is difficult enough that it almost certainly takes someone with Ormes' "carefully graduated experience" to do it unprotected. Following on such a route can be a different matter: relative novices, or those hikers/climbers used to mostly 2nd Class routes, or those who get rattled easily on any kind of steeper or uncertain slopes, often do much better with a belay on 4th Class terrain, if the look of it alone doesn't stop them.
This take on the situation does pretty much correspond to the common facing in/facing out on descent rule of thumb. (Better yet, use a quick, sport rappel for the diciest parts of 4th Class routes, which beats tedious, exposed downclimbing by a mile.) As well, this angle on things lines up fairly well with the fatal fall possibility rule of thumb also, with whether the exposure/traction ratio is high enough for a fall to be both severe and within the conceivable realm of possibility -- i.e., enough of a concern to detract from the enjoyment of the climb.
And beauty will go savage in the secret mountains. - Robinson Jeffers