I am new to the phonegap. We are developing our first app using Sencha Ext JS, and planning to build native application for android, and iOS using phonegap. How do we know the compatibility of each phonegap build to these platforms. Does the latest phonegap build support android 4.x and iOS 4s? Are there any limitations?
See https://build.phonegap.com/current-support and then cross reference that with the documentation available at http://cordova.apache.org/docs/en/latest/guide/support/index.html (navigate to each platform in question in the sidebar for specifics)
For iOS, the minimum version expected is iOS 8, which is available on iPhone 4s and better. Android support goes back to API 14 (4.0+). For Android, I would definitely use the Crosswalk plugin because you'll have a consistent chromium environment, but beware that they are deprecating support for 4.0 -- see https://crosswalk-project.org/blog/deprecate-40.html