"4 GB cards can support the autoload feature if they are formated in FAT16 manually. ... But be aware that the formatting of 4 GB cards in FAT16 leads to non-standard 64K cluster size."
A file system with FAT16
was originally designed with a max. cluster size of 32 kB, so the maximum partition size was 2 GB, this is the 'original FAT16 standard'.
With Windows NT Microsoft increased the maximum cluster size to 64 KB, so a partition size of 4GB now is possible.
But such 4 GB partitions may not be supported by all devices or operating systems.
Since MS Windows NT / Windows 98 most OS can use this 'big' partitions without problems, also all Canon Digi-cams with SD-card storage support can handle such 4 GB partitions well.
On other operating systems like DOS or older Windows versions there may be incompatibilities.All CHDK-supported cameras can use 4 GB SD cards with the FAT16 format without problems, actual operating systems like Windows XP/Vista/2k3, Linux, Mac OS works flawless, too.