The iPhone 4 cannot run iOS 6 in 2024 as apple stop signing iOS 6 for 4 but for IPhone 4s the community managed to find a work around to get iOS 6 to work
The iPhone 4s can be downgraded to iOS 6.1.3 while the regular 4 can not (at least not perfectly, while the 4 can be made to run any version the flat design bootloader with Apple logo and battery flat symbol will always remain with the currently available methods)
and you can tell by the model number at the back of the phone with A and a 4 number digit. When you search the model number on the internet it gives you the model of the phone
iPhone 4 was only supported to ios 7 Meanwhile the iPhone 4s could support to ios 9 but made more slow The iPhone 4s in this video was running ios 6 bc it ran fast and more responsive than newer versions of ios. And also you can tell the difference by lines for antena.