Published Unity app won't download on iPhone 4/4S

We recently published our first app and it got reviewed and accepted and is now on the App Store.

However, it appears that people with an iPhone 4/4S can't dowload our app, despite being above our iOS version requirement (7.0). The App Store gives them an error instead : "This app can't be downloaded at this time". It otherwise installs fine on all kinds of iPads and iPhones 5/6.

The app was made with Unity 5, then built with Xcode 7.1.1 on Mac OS X Yosemite, The min. iOS version was set to 7.0. During our Testflight period, it installed and run flawlessely on iPhones 4/4S.

I've tried looking around for people with similar problems but wasn't able to find anything. Is there anything in our Unity publish settings that could have caused this to happen?

Thanks in advance.