Sorry to hear you guys are having so many problems with v4. I've made five different map installs in testing and each one did what seemed like the logical thing.
My maps are installed on the SDcard and I have given them my own file names. Using the Advanced option I installed additional maps one at a time and each time MI just installed the new map with the name I had given it in BaseCamp on the SDcard. I presume that's because I didn't have any maps on internal storage. MI did not erase and rewrite my existing maps on the card. Since I wasn't doing anything to the existing maps MI preserved them on the SDcard.
The one thing that does have me puzzled is how MI can change the properties of the GPS. I was under the impression that a Garmin GPS could only index a certain number of map segments - on my Montana that number is 4000 segments - so why would MI allow us to load more than that number?
Well hopefully v4.0.1 will come along soon.