I use a Zumo 550 and have the NuMaps lifetime update and think its good value as it keeps the PC maps & the GPS maps constantly up to date. Mind you it takes eleven hours to do it generally on my broadband connection. Generally start downloading before I go to bed and finish it up the next morning ......If your on a dial up Internet connection....forget it.
The only issue that may arise with the 400 is that I think it has a smaller internal memory, so I would check on this with Garmin before signing up.
Maybe somebody on here with a Zumo 400 can advise you better on this memory issue than I can.
The lifetime map updates only applies to your particular GPS unit for its lifetime and is not transferable to another unit.