How difficult is it to write an Android app that works with Android 4.1, 5, 6,7, 8 and the Android 9 which will be released by September?

Pontifications

Serious and honestly not really a work related question:
- How difficult is it to write an Android app that works with Android 4.1, 5, 6,7, 8 and the Android 9 which will be released by September?
- Is it better to write multiple apps, e.g. one for 4.1 and one for 5,6 and one for 7, 8 and 9?
How much Android functionality do you lose as you move from Android 9 down to 4.1?
I’ve been supporting Android apps since 2012 so I know the answers to most of the above questions, but maybe there’s somebody out there who can give me a 2018 answer.