You're right. Format 4 contains all of the individual pieces of information that could possibly be stored in any of the other formats.
However, it is a total rewrite of the file structure. It does not have any retro engineering connection to format 0 or 1. Think of the large number of files out there that simply have a .ild on the ends of them. What does that mean, now?
Format 2 has been a part of the published standard for many years. For some reason it was not universally implemented; or at least not as accurately as formats 0 & 1. As you know, the 62 color palette is assumed in many cases.
If an ILDA file reader was designed to scan through a file looking for the string "ILDA" , three binary zeros and another byte, 0 or 1, then it would be possible to open a file that had format 2 data in it and the reader would be able to skip that and still get all of the vector art from the file. So if a reader was designed to skip 2, it would just as easily and accurately skip 3; even though the reader may have been coded before 3 was even an idea.
On the other hand, if we forget about 2 & 3 and go straight to 4 & 5, we are certain that the older ILDA file reader will get nothing, because there will be no format 0 or 1 to find.
There is also a loss of information in the form of not using format 2 and going to 4 or 5.
In laser vector art, palettes are always in play. Even if you have a frame with 1057 different points of unique colors, that is still a palette of colors. If there are less than 256 colors, then it should be indexed with the single byte in format 0 or 1. If there are more colors than 256 it makes sense to use the natural order of the vectors as an index into a color table.
The use of 2 and 3 must be connected to the understanding that a format 2 palette can stand for a whole series of vector frames that follow it, until another format 2 or a format 3 is encountered. But a format 3 color table must always have a one to one relationship with the next format 0 or 1 that follows it.
By format 3, I mean with the same, standard, section header that all the other formats have.
If you make a set of frames that have less than 256 unique colors per frame, then, the palettes themselves, separated from the vectors, are an important part of the art to convey. If you save all the frames as 4 or 5, you have no separation from the vectors and the palettes. How do you get the original palettes back?
I understand why formats 4 & 5 exist. But should they really be called .ild?
And, if formats 2 & 3 were implemented instead, then why would we need 4 & 5?
James.