It sounds like you are approaching building a watermaker from the wrong end. You don't say exactly what you are doing, but nobody searches for random dimensions in membranes for their watermakers.
If you get a standard size membrane - 2521, 2540, 2580 - these all have numerous sources for pressure vessels suitable for seawater RO. I don't have any experience with the 4" sizes.
I just bought 2 Dow Filmtech SW30-2540 membranes from AmericanRO.com for $175 each. This is the lowest price for those membranes I know of. They also sell housing for all of the common sizes: SEAWATER HOUSINGS PRESSURE VESSELS
Again, you asked about 2.5"x21" membranes, as well as 4"x40" membranes. This suggests that you might not understand watermakers, or what you really need. One cannot just connect any random size membrane to any random volume pump and have a watermaker that works. A larger membrane does not produce more water than a smaller one if you cannot pump enough water at pressure through it.
I guarantee that if you found a 4" membrane of any length with a suitable pressure vessel for $500 that is is not for seawater RO. You need to understand this difference if you are building a watermaker.
If you describe better all of the watermaker components you already have, it would be easier to provide help.