Looks like the m4 version is only 1080P and without parking mode, but the m4 plus version is 1440P and includes parking mode. Both feature the Sony sensor, and replaces the startup message from ": Ding Dong : hello ddpai" to just " : Ding Dong :" (Not sure about the other audio messages, can anyone confirm if they're in English??). Both have a sexy black (matte?) finish compared to the shinny brownish finish on the m6 series. M4 has a black ring around the lens vs the red ring around the M4 plus (like on the m6 plus), don't care for the red, would rather have it all black and more discreet.
I'd really like to get my hands on the M4 plus but they don't appear to be sold anywhere yet, perhaps they're still actually developing it and gearing it towards English-speaking customers? That would be huge as the importance of dash cams is starting to pick up more globally. If they worked out the connectivity issues (Android) and made the app more user-friendly I'm sure their sales would explode.