iOS 7.1 on the iPhone 4: As good as it's going to get

You can't patch out a slow processor, but Apple does its best.

iPhone 4 - Wikipedia

iOS 7

Apple releases iOS 7.1 with CarPlay support

iOS 7.1.2 Update Released [IPSW Download Links]

If You Care About Security, Throw Away Your iPhone 4 Right Now

A few of the phones in my collection, from left to right: iPhone 4

Download iOS 7.1.2 For iPhone, iPad [Direct Download Links

iOS 7.1 on the iPhone 4: As good as it's going to get

Final Words - Apple iPhone 4S: Thoroughly Reviewed