I was hoping that someone familiar with those code sections would reply. If it was my job I would provide a support so that the unsupported part is no longer than 3 feet. That, to me, would seem to satisfy everything. It seems to me that the 4 ft length mentioned is the total length allowed under those circumstances period. And since support (or lack of it) is not mentioned I would say that the provisions of 350/351 would apply and that 3 ft maximum unsupported length would be the rule.
Under 300-1(a) it says "This article covers wiring methods for all wiring installations unless modified by other articles"
I would say that 350/351 has the last word.
Any other opinions on this?