Ipad Minimum Requirement Swift 4

From this Official Apple blog post:

Simply put, if you write a Swift app today and submit it to the App Store this Fall when iOS 8 and OS X Yosemite are released, you can trust that your app will work well into the future. In fact, you can target back to OS X Mavericks or iOS 7 with that same app. This is possible because Xcode embeds a small Swift runtime library within your app’s bundle. Because the library is embedded, your app uses a consistent version of Swift that runs on past, present, and future OS releases.

So, Swift works with any device running iOS 7 or later. That includes every iPad model except the original (long gone) 2010 model.

If you are talking about Swift 4.x only, that would be Xcode 9.0 and iOS 10.3.1, but as noted, Swift is backwards compatible otherwise.

The iPad 2, 3 and 1st generation iPad Mini are all ineligible and excluded from upgrading to iOS 10.