As Strephon Alkhalikoi said above you can combine several maps in one gmapsupp.img file in your sd-card. This can be made with i.e. gmaptool or when transferring the maps to gps from Mapsource. A new gmapsupp.img is generated each time from Mapsource then.

I think that the nuvi200 have a limitation on how large a map file can be that is 2 GB. One workaround then is to generate the first gmapsupp.img and the rename it to something like gmapsupp.img.CNEU and then generate another gmapsupp.img file. You can then rename the wanted file to gmapsupp.img and the other file to something else when hooked to the PC and make a relatively quick switch between maps (provided that you have access to a computer or a smartphone capable to reading and renaming files in the sd-card) In this way you can get some use of your 4 GB card