for some reason i cant download zwift on my ipad 4 (A1460) - it says not compatible

Maybe because your iPad isn't compatible?

Here's the compatibility list (from the below-linked iTunes App Store) where the iPad 4 is not mentioned as being compatible:

Compatibility: Requires iOS 9.0 or later. Compatible with iPhone 5s, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone SE, iPhone 7, iPhone 7 Plus, iPad Air, iPad Air Wi-Fi + Cellular, iPad mini 2, iPad mini 2 Wi-Fi + Cellular, iPad Air 2, iPad Air 2 Wi-Fi + Cellular, iPad mini 3, iPad mini 3 Wi-Fi + Cellular, iPad mini 4, iPad mini 4 Wi-Fi + Cellular, 12.9-inch iPad Pro, 12.9-inch iPad Pro Wi-Fi + Cellular, 9.7-inch iPad Pro, 9.7-inch iPad Pro Wi-Fi + Cellular, iPad (5th generation), iPad Wi-Fi + Cellular (5th generation), 12.9-inch iPad Pro (2nd generation), 12.9-inch iPad Pro Wi‑Fi + Cellular (2nd generation), 10.5-inch iPad Pro, 10.5-inch iPad Pro Wi‑Fi + Cellular, and iPod touch (6th generation).

Zwift on the App Store

Hi. When I read the lengthy compatibility list for Zwift, it lists only 64-bit devices, which your iPad 4 is not. That's probably the reason.