Thinking that ANY diet plateaus after 3-4 days is idiotic, at best.
What happens with a low carbs diet is that during the initial 3 days or so you deplete muscle glycogen. Since each gram of glycogen is stored with 2.7 (let's say 3) grams of water and that an average body might store 400g of glycogen in the muscles, it is fairly safe to assume that one will lose up to 1.6kg (3.5lbs) during a depletion and this is only water and stored carbs.
Add to that that you might lost anywhere from 0.5 to 1.5lbs of fat during those 3-4 days and it gives you a fairly quick drop in body weight of 4-6lbs.
Obviously, once you are depleted you don't have much ''rapid weight'' to lose so the rate of body weight loss slows down... but NOT THE RATE OF FAT LOSS.
You might lose 4.5lbs during the first 4 days (3.5 from water and glucose, 1 from fat) and 1 during the last 3 days of the week. This gives the illusion of a slower loss, but the rate of fat loss is the same.
So saying that fat loss will plateau after 3-4 days of low-carbs dieting doesn't make much sense.
And everytime you load up on carbs THEN switch back to a low carbs diet you will once again experiment a very rapid drop in body weight, simply because you once again have glycogen and water to lose. So it is fairly easy to assume that after 3-4 days your loss hits a wall and after a day of higher carbs it starts again.
If someone uses a low-carbs diet (without a significant carb-up) for an extended period he will lose a lot of ''weight'' the first week, a lot less the second week and still less for the weeks after that.
It might look like this:
Week 1. - 6lbs
Week 2. - 4lbs
Week 3. - 2lbs
Week 4. - 2lbs
But in reality you are likely losing the same amount of fat every week, but more water and glycogen during the first week.
From experience, in the long run, all ''smart'' diets if properly followed will yield roughly the same rate of fat loss. While initial losses might be relatively rapid, on the long run a loss of 2-3lbs is the norm with most good dietary approaches.
The problem with low-carb diets is not so much that you hit the wall sooner than with other diets, but rather that the sudden drop in bodyweight during the first week creates unrealistic expectations as to what can be achieved in the long run.
Obviously this simply relates to fat loss. The issue of energy also comes into play. Staying on a low-carbs diet can lead to lowered energy levels, thus decreased workout quality.
IMPORTANT POINT: If someone cuts down his calories or energy intake too much he will subconsciously try to expend less energy and will reduce his level of activity (e.g. walking at a slower pace, less fidgiting, less walking, becoming lazy, etc.). This decreases energy expenditure... if you use less energy you will lose less fat.
This is why WHEN ADVOCATING A LOW-CARBS APPROACH (which I don't use all the time and with everybody) I recommend ingesting carbs every 7 days (more often if someone is already lean...the leaner you are the more often you need carbs). The quantity to ingest will depend on the individual's tolerance for carbs and degree of fatness. It could go from one meal up to one full day.