@softstuff
In clarification, a 4'GB' [gigabytes] card will have somewhat less than 4,000,000,000 bytes available for storage [see full explanation [Only registered and activated users can see links. ]]. The so-called garmin image size limit is in fact a file size limit imposed by the FAT32 file system and it's actually 4GiB [gibibytes] less 1 byte, i.e. 4,294,967,295 bytes. So if you have a really big chart approaching 4GiB [gibibytes] in size it simply cannot fit on a 4GB [gigabyte] card anyway.

Part of the secret to dispelling the misunderstanding about these 'size limits' lies in using the currently accepted terms [gigabyte for decimal and gibibyte for binary when talking about drive memory], as well as the correct widely accepted standard abbreviations [e.g. KB, MB, GB, TB for kilobyte, megabyte, gigabyte, terabyte and similarly KiB, MiB, GiB, TiB for kibibyte, mebibyte, gibibyte, tebibyte]. Using nonstandard abbreviations can be really confusing, for instance Gb is actually short for gigabit [but i do it myself lazily sometimes] so it's clearly wrong when used for gigabyte and while saying 'gig' verbally is quite universally acceptable for gigabyte it's therefore perhaps an almost acceptable 'non-standard' written abbreviation.
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I really believe card size in itself would not be a problem for your plotter, but it's quite pointless using bigger than 8GB anyway when it can only use one image not larger than 4,294,967,295 bytes [~4.29495GB]. Regardless, most charts will fit on a 4GB card