Ionic 2 supports Android 4.4 and up, if you need to support older versions you could use Crosswalk:
Android 4.0 - 4.3 accounts for only a bit over 20% of the usage and will probably drop significantly in an year.
Well, this depends on the app size. AFAIK Crosswalk adds around 20MB to the APK size. Moreover many apps are taking this approach and the users are more or less used to it already. For all performance improvements and standards compliance that Crosswalk provides, IMHO it’s still a reasonable compromise. If you have app size constraints then you can take a look at Crosswalk Lite - it’s half the size but might be more difficult to install/configure.