About the 4 GiB barrier!
Well, you can put as many <= 4 GiB map files as you want in the Map directory of your memory card. As long as there's room for all the files anyway.
The problem is other constraints like how many map tiles your unit supports and the most severe constraint about the number of map families it can support.
If I store all TOPO Norway Adventure in one gmapsupp.img then I have 20 map families there. That's NOT readable by any Garmin unit I've tested including the Garmin Montana 650t. Normally it works with 16 or 17 MapIDs as the total for all maps but not with 20.