Is the iPhone 4 still suitable for development purposes?

I’m scouring eBay for an iPhone to test my app. Ideally I’d like an IPhone 4s because the version of iOS is newer (and it’s cheaper to buy second hand), but I wondered if I can get away with an iPhone 4.

In regards to testing and building ionic apps, should I go hunting for an iPhone 4s or newer, or would an iPhone 4 still be suitable?

The app is rather comparatively basic (uses JustGauge for some gauges / dials, internet for the gauge data, compass plugin, brightness plugin, localstorage and perhaps notifications), but I want to ensure that if I write and test for the iPhone 4, then I’m not going to hit any problems targeting the iPhone 4s and newer (other than screen size) and likewise, if I’m going to buy an old phone, that I can test and debug on it.