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If I had to guess the dimensional weight is applicable when the packaging has weird stuff done to it. Examples I have seen include: Taping together multiple express boxes into a large Frankenstein box. Forcing an oversize object into the box which drastically changes its size. A FedEx Tube with an object that is too long for it that has the parts sticking out covered with more boxes or tape. Slicing the box and re taping it in an unintended fashion (Ive seen a large object with several cut up and flattened FedEx boxes taped to the outside). The FedEx Pak is basically a bag so it takes the shape of whatever is inside, dimweight wont apply to these either as long as they are under 940 cubic inches per the service guide. I think you will be fine if the box you are using stays in its original shape, we just have to be ready for those packages that try to abuse the exception. More on reddit.com

The official guidance when shipping a tube or their triangle "tube" mailer is to measure it as if it was a rectangle. So you are right in the 4x4x50. I believe there is an oversized box charge past 30" on any length. More on reddit.com

I know FedEx has boxes that fold into a triangle, that we use at work for drawings and the occasional poster. It comes flat and you fold it into a triangular shape with adhesive already on the box. More on reddit.com