Here is a demo project to reproduce unstable behavior - https://github.com/kanoshin/cordova-bug-reproduction. You can try to run this app ~20 times and I'm sure that you will be able to reproduce huge memory leak and 100% cpu usage (both on real device and emulator). This app shows current state on the screen - "Done!" means that everything is fine, any other state is most likely a marker of 100% cpu usage.
This app is using 3rd party plugin that works stable on previous versions of cordova-ios. After some research I noticed that unstable behavior disappears if I wrap all plugin calls in:
block. Otherwise WebThread sometimes goes crazy and device becomes unresponsive. I believe that this is a bug because previously it was ok to run plugin calls that takes small amount of time in the main thread. Also it seems that a lot of existing plugins don't care about running in background thread and it makes update to cordova-ios@4 impossible.