Windows下的变更不大，主要还是bug 的修复，性能的优化，API的扩展实现。
变化最大的是在Mac上的那个Xamarin.iOS build host 。那个配对的代理软件，消失了！！！！！！！！！！！！你他么是在逗我啊！！！！！！劳资连夜升级，熬夜找那个build host ，你他么说升级消失了！
官方的意思是：那个代理软件太麻烦啦，换成自动化的SSH登录识别，MAC机器一次性开启远程登录，后续的客户机只要有对应的登录指纹就可以连接了！！
相当于在VS中，进行了远程登录搭配Mac作为的服务器！！蛋疼哦！~~~
然后以Mac的管理员用户，登录连接MAC.....................
由于跟以前登录方式不一样了，所以，只好硬着头皮一点点的扒拉。。。
官方原文：https://developer.xamarin.com/releases/vs/xamarin.vs_4/xamarin.vs_4.0/
https://developer.xamarin.com/guides/ios/getting_started/installation/windows/connecting-to-mac/
那如果还想用3.0的build host的 ，请看这里：https://developer.xamarin.com/guides/ios/getting_started/installation/windows/troubleshooting/
Xamarin 4.0
Xamarin 4.0 updates Xamarin.iOS 9.4 and Xamarin.Android 6.0 releases. This version also introduces the new Xamarin Mac Agent which replaces the old Xamarin Build Host with a new approach taking advantage of the built-in MacOS Remote Login feature based on SSH for a faster and more reliable connectivity.
Xcode 7.2 support
XamarinVS 4.0 now includes support for Xcode 7.2.
Xamarin Mac Agent
Our new design focuses on avoiding the pain points of the original Xamarin.iOS build host that we are no longer shipping.
Key Benefits
Simplicity and Security
- We have done away with the PIN-based Mac pairing process. The new Xamarin Mac Agent only requires that Remote Login (aka SSH) is enabled on the Mac.
- The new Connection Manager in Visual Studio will discover, authenticate, and remember your Mac. Moreover, since all communication is tunneled securely through SSH, you only have to open one, well-known port (i.e., port 22) on your router or firewall.
- There’s no longer an app to start on the Mac. If you have a compatible version of Xamarin.iOS installed, VS will automatically deploy and start the new agent.