Probably 4.1, that's my current minimum build target.
Yeah, you'll have to buy the app for Android again, I've no way to carry over licenses from iOS to Android. Given the amount of cash and time this project has sucked up thus far, I'll need that income
I've learned a lot though... for example that developing for Android is quite a bit harder in some ways, even when using a platform like Xamarin. Memory management, threading and performance are pretty much non-issues on iPhones, on Android these things often need careful attention.