While people are looking around for improvement in macOS by Apple to fix the battery drainage issue on new 2016 Macbook Pro. However Apple did opposite to that and released iOS 10.2.1 beta 4.
Since it is beta, it is only available for developers. The strange thing is that just few days ago, the fruit company rolled out iOS 10.2.1 beta 3 and now we have another beta.
This makes us believe that there might be some big bug in iOS 10.2.1 beta 3 that Apple noticed in first glance and released the updated version. Other reason might be the bug could be used by jailbreak community to make their way through the latest iOS 10.2.1. So Apple played smartly and closed it before the public release.
Anyways, macOS and iOS are now common in terms of their beta count as we already have macOS Sierra 10.12.3 beta 4 released few days ago. Now we have iOS 10.2.1 beta 4 too. Overall, iOS 10.2.1 is a small update and it will bring some performance improvements and bug fixes to supported iPhone, iPad and iPod Touch. As already discussed, the beta is only available for registered developers.
Download iOS 10.2.1 beta 4 IPSW
So if you are registered, open Settings app on your compatible iDevice and go to General -> Software Update so get the latest iOS 10.2.1 beta 4.
Apart from that, if you want to download .ipsw file, head over to developer.apple.com and sign-in with your credentials to get the latest files.
Once downloaded, you can install it using iTunes latest version on your compatible iPhone or iPad.