However, you will still need the mapping. Mapsource is a program that you use to handle the mapping and plot routes using the mapping. The mapping data is not part of Mapsource but a different entity. Not a great analogy but it's like MS Word (the program) and a Word document (the data). Neither is any good without the other. In the case of Mapsource the program is free, the mapping data has to be paid for.
The mapping data already on the GPS isn't transferable to your PC unless you're using Basecamp, even then it's not ideal and can take quite some time requiring you to plug the Zumo into your PC and transfer the mapping temporarily across. I don't think it stays on the PC, it actually reads it off the GPS as you use it.
Depending on the age of the mapping on your 400, and how long you intend to keep the GPS, you might want to upgrade the mapping to a Lifetime subscription. This can be done for around £50-60 (it's gone up) by buying a Garmin gift card from
Handtec or
Pentagon GPS. The Lifetime bit refers to the GPS, not you. Sell, loose or have stolen and it's gone with the unit. If Garmin repair or replace it they should transfer it though.
Once you have the subscription set up in your myGarmin account you can update every 4 months or so as the new mapping gets released. From there you can download to your GPS and/or your PC. Once on your PC you can used Mapsource and Basecamp to your hearts content with decent load times and without the GPS plugged in.