Tell us what you are trying to stuff into this box.

I am trying to install multiple boxes, some will have a single 14/2 NM cable, some will have 2 cables (1 in 1 out) with internal clamps, 1 will have 2 cables and a bell transformer wires.... I know how to calculate the required box size, and I know that 4 inch by 1 1/2 inch octagonal boxes will be just fine. However, I can't find any 4 inch boxes. All the boxes I can find are 3 1/2 inches across, even though the store label says they are 4 inches....

Don't worry about what the actual measurement is, the boxes are listed as 4x1.5in... this is a standard "trade size" and the volume is 15.5 cu.in. If the volume of this box is adequate, you can use it.

Here is an another question. Since Table 314.16(A) of the NEC mentions 4 inch by 1 1/4 octagonal boxes, but nothing smaller, why would HD and Lowes not carry the 4 inch boxes, but only 3 1/2 inch and smaller?

There are many electrical components that are available in electrical supply houses that are not routinely carried by the big box stores. I'm still not clear why you think you need really large boxes with your modest box fill requirements.

Octragon boxes and round boxes only come in one size that have seen. The size that fits under a light fixture or fan properly. Anything bigger wuld hang out of the cover.

kudzu9: I simply wanted to use a box that is listed in the NEC table, it is not that I want a really large one - in fact I want the mid-sized one because the smallest one is too small....

normel: Thank you for the explantion - to make sure I understand correctly, the NEC indicates 4 inches, but in fact this is a 'trade size' and the actual box is 3.5 inches accross (interestingly it is close to 4" diagonally)

ecranny-
The boxes you buy at Home Depot are code compliant. You just need to know the cubic inches of the box and comply with the box fill numbers for various gauges of wire, etc.