Pokemon Go isn't compatible with Samsung Galaxy S4 any more?

I'm not sure what happened originally, but it's a bit of a moot point now: As of July 2019, Niantic has updated their supported devices and operating systems, dropping support for Android v4 (Kitkat) (Twitter Announcement, Support Article), as well as iOS 9 (Support), and the Apple Watch (Support):

The currently supported technical specifications are as follows:

Android

• Android 5 or above
• Preferred resolution of 720 x 1280 pixels (Not optimized for tablet)
• Strong internet connection (Wi-Fi, 3G, or 4G)
• GPS and Location Services
• Rooted devices are not supported

iOS

• iPhone 5s or above
• iOS 10 or above
• Strong internet connection (Wi-Fi, 3G, or 4G)
• GPS and Location Services
• Jailbroken devices are not supported

See Niantic's 'Supported Devices' Page for more information.