Difference between Mono.Android.Support.V4 and Xamarin.Android.Support.V4-r18

Hi, like in subject, i'm a little confused about components Xamarin.Android.Support.V4-r18 and DLL Mono.Android.Support.V4. In my project, only for ICS, i would create a ViewPager with a DrawerLayout, but ViewPager is only available in Mono.Android.Support.V4, while FragmentPagerAdapter is available in Android.App, Android.Support.App.V4, Android.Support.App.V13.