You're not using a PGB-specific Crosswalk plugin, so you might want to try one of those first. See Plugin Search - Apache Cordova. You're probably building for ARM at the moment, so add the "arm" version of the plugin. It probably won't make any difference, but someone else had a problem where using the non-PGB plugin was resulting in downloading the Intel APK instead of an ARM APK, which is one more headache you probably don't need.
On to your issue:
Can Android play the video if you copy it to the device on its own? Perhaps there is an issue with how the video was encoded.
If you can, I would look at the device's console logs (adb logcat, or there are apps in the Google Play Market) and see if there is anything interesting when you play the video from your app. That might help troubleshooting.
If you have no success there, you might want to search the plugin repo for a native video player.
One last thing: don't use the <gap:plugin tag> -- use <plugin> instead. This is unlikely your issue, though, but it's important to have a modernized config.xml.
Thanks for your reply - I checked out the documentation of the arm/intel version but both give the same entry to place in config.xml for PGB"<gap:plugin name="cordova-plugin-crosswalk-webview-pgb-adapt" source="npm" version="1.4.0-dev-5"/>"
The video is encoded correctly and the device does play if when the browser is used.
If I use a device with android 5, the phone gap app even plays it!
You'd have thought the desire to play mp4 videos would have been high on the list of things most people would want to use an app for! Not exactly like I'm trying to use a VIDEO element to do something other than play a video.
Video on old Android versions is a pain in the you-know-what. I'm surprised Crosswalk isn't working in your case, but versions of Android < 5.x like to throw curveballs.
First thing I'd do is to make sure that your app is indeed using Crosswalk. For one, your APKs should be larger than you might otherwise expect (due to including a build of Chromium). Second, execute a "alert(navigator.userAgent)" and report what comes up on each device/OS version.
Finally, video itself is a pain in the rear, regardless of PhoneGap or not. Worst case, I'd suggest using a plugin from the Cordova plugin repository -- those work better on older versions of Android.