When you have a million users, 70,000 of them is a pretty big number to neglect. edit: numbers
Usually older devices are on older OS because they can't update. If they can update and haven't then it will probably be because they don't know how or because they don't want to.
If your app needs features in iOS 6 to run, then so be it. If not then make it runnable on iOS 5 and 4.
If your app used to work on iOS 4, but some new features require iOS 5, then your old customers won't get any more updates but should still be able to use the app. Those stuck on iOS 4 wanting to buy your app won't be able to.