This is different. Moto G 2014's hardware is capable of running Marshmallow even though it didn't receive the update yet. The iPhone 4 too old. If iPhone 4S has difficulties running newer software due to hardware limitations, the 4 would likely not even be able to run it at all.Same can be said for Apple not updating iPhone 4 to iOS 9? Is Apple neglecting to path major vulnerabilities? At least with Android, you have option to flash custom ROM with updates? Can you do that with iPhone?
My Moto G 2014 have not receive marshmallow update yet, but I flashed CM 13 with Android 6.0.1. Can you flash iOS 9 to iPhone 4? Nope, too bad, you should upgrade to iPhone 6S. With Android, I always have option to flash latest custom ROM.
Try finding a older budget phone like .. . a Samsung Conquer 4G. Look at the specs and decide if flashing a custom ROM would work.