You still need to write apps to be compatible with 2+ year old OSes if you want them to be able to install those apps. You don't get infinite forward compatibility for free. There are many updates I stopped being able to get on Android 2.x after 4 was released.
The market share of <Android 4.0 is small enough now to ignore, especially if you are also ignoring iOS entirely. Definitely not enough to stop you from writing a webapp in the first place.