I don't know that there is anything technically wrong with doing it that way. I suppose, depending how the wort is run off, you could have 4 boils with 4 different gravities, which would make hop utilization somewhat unpredictable. The biggest downside is that he's doing 4 times the work and 4 times the cleaning. Wouldn't it be cheaper to buy a single, adequately sized BK, than buy 4?