How to specify a specific app to always use 4G/data even when on Wi-fi?

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http://forum.xda-developers.com – For example, I use Talkatone or Sipdroid for my phone calls so I don't want these apps to get briefly disconnected when i connect to a wifi network because i may miss an incoming call. Plus, since voice calls use such little data and I'm almost always have strong 4g signal, it seems like it would make sense to just keep calls on 4g all the time. It's also annoying if i'm at home in a call over wifi and need to go somewhere, then i have to call the person back. Any ideas? I'm using an un-rooted Galaxy S3 t-mobile currently, but will probably root in the future if absolutely needed when i (HowTos)