With the file extension AI4 so-called general encapsulated postscript files of the vector graphics software Adobe Illustrator were marked. It is the default file format used to store, among other things, drawings, objects, and settings. The file extension AI4 refers to the version 4, which came in the early 1990s only for Microsoft Windows on the market.
The AI format does not save its data in EPS format and AI4 files may contain some Illustrator-specific instructions, so AI4 files may not be able to be opened by EPS-compatible software.
In addition to the meanwhile common ending ai from the version 3 to 8 also the ending ai with the appropriate version number was used. It should be noted that each version has also introduced new functionalities and other keywords for the header information. Therefore, ai files of different Illustrator versions are not necessarily 100% compatible. However, most recent programs that can handle Illustrator files can handle all the old format variations.