Bob
6x4 is the capacity, physically speaking. The width is irrelevant, you just have to keep going (in the same way that it is not harder to plane a long board than a short board, but it is harder to take a thick shaving than a thin one). But the higher the stock, then yes, the more effort is required. The skewed blade makes that much easier than it would be if it were straight, of course.
Remember it is a trimmer, not a saw.
S