I fitted a sumvision one to mine (connected to the pumpkin AV in), wasn't the 4, think it was the 2+ so doesn't have miracast.
No problems with it all and hardwired to a 5V converter is about as discreet as you can get. Mine is stuck with double sided tape to the top of the little cubby hole below the steering wheel, where my toll card reader used to be.
Not sure if the 4 version is quite that small though and the port locations might be an issue - seem to remember that is why I choose the version I did, power jack on the back and the card slot on the front. Of course it doesn't do miracast, so if that's what you are after it's not an option anyhow
It's pretty small Simon but has the SD & USB slots on the side Miracast will be good for me when streaming Eric The Car Guy & MCM videos when I'm parked up at MaccyDs (instead of eating up my hotspot allowance), but the youngest won't be able to stream from her iPhone. Maybe that's a blessing though!
Both works the same for movies and stuff, I bought M8S which I am now mostly using in my house becasue which was the sole purpose. I wanted a box with KODI so I can use in both my car and at home. If you just want to use in the car Cyclone 4 is perfect, it has miracast and it is small in size too, no troubles whatsoever, 5 mins to install, no extra adaptor/wiring required. Let me know if you need any more help. For the link, just type M8S in Ebay and you will see loads, here is the link too