There are a range of security issues with iOS 7 and below that were resolved in iOS 8. Mumba has been performing research and have found that from a security standpoint, it is best to only support iOS 8 upwards. This will unfortunately impact iPhone 4 users because they’re not able to upgrade to iOS 8 and above. You can Google these issues as they are widely known and here is an article for example: https://www.intego.com/mac-security-blog/if-you-care-about-security-throw-away-your-iphone-4-right-now/
Mumba has setup the app for iOS 8 and above and Mumba would suggest, to maintain best practices, that we retain this minimum standard going forward. For those users on lower versions it would be best to use the browser version instead and save a bookmark to their homescreen that quickly opens the page.
The minimum Android version is currently 4.0.0 however there are many versions in the chain (that go up to the current version 7) so it is very possible that a particular version (for example 4.2.3) may have a known security issue that the installer will reject so Android is a little murkier. If there is a particular version that is having trouble Mumba will need to investigate it on a case by case basis but overall it should be fine.
Mumba would suggest that it is compatible with 4.0.0 and above however if the user has issues to report the issue and version number and to use the browser while Mumba investigates.