A short story and background :
I have been a flying a phatom 4 using a samsung galaxy tab 3 tablet 7" with android version 4.4.2  for a year now without any problem.
Two weeks ago I upgraded the firmware in both drone and controller, and also upgraded the DJI GO APP from version 2.8.4 to the latest DJI GO 4 APP
Since the upgrade, the video feed became absolutely useless, the ​image​ is​ frozen , pixel​l​ated, ​with ​colo​u​r stripes, like watching a coded tv channel without decoding the signal. However the videos and photos in the sd card are ​fine.
So I contacted DJI online technical support and after almost two hours explaining the problem and trying the only solution they gave me, which was to reset the firmware to factory defaults, and to reinstall it again, which it did but it did not solve the problem, they told me, to my surprise that I had to send the drone to a repair centre !

support: still the same issue?
me:  yes
support: I see you really have to send that to the repair center to check what happpens to the camera
me :   send it where ?
me :   but it was working ok before the upgrade. How do you explain that ?
me :   dont u think the software is the problem rather than the camera ?
support: seems to be the hardward there is something went wrong inside the camera you need to send it to the repair center so they can review the drone, where are you located?

So I went to the shop where I bought the drone and showed to the guy there the problem in my tablet, he then unplugged my tablet and plugged in his iphone 5 , and everything was sharp and clear, perfect video feed. Nothing wrong with the camera.

Question:
Now what ? Do I have to buy a new tablet ? Which one would be best for less than 300 eur if any ?

Soon as I read your post and saw "Samsung" I knew it was bad.

More  often than not, I find my Apple iPad Air 2 + Cellular will work better than an Android with DJI GO.  I've given up on the Samsung brand as they add too much background software than competes for resources with the processor and it break up the video feed, imho.  Yesterday Goggle added some health app thing to the Samsung which Samsung already has that does work in the background like counting your footsteps etc. but I can't find what Google did or where that app went.  Lots of redundancy is running on their Android/Samsung setup.

Apple just came out with some cheaper tablet than the Air 2 or Pro tablet too, but I don't know if it works with GO.  I've learned the hard way to stop buying cheaper Samsung Android devices that don't work- or soon won't - and buy the one Apple device that does work.  Also, the 9.7" screen size is better if you want to use a autonomous app like Litchi or Autopilot as the smaller screens are too tedious to set up flights due to small screen working area.

Good luck!

We are so sorry to hear about your unpleasant experience, the mobile device connected to the drone would make a big difference in some situations, the online support should have considered it as a factor which may cause image transmission issue and help you do some troubleshooting before making a conclusion that it's a hardware issue.
Based on what you described, it seems the phone is not compatible with the drone, to guarantee the performance and experience, it's recommended to use the devices in the list below:
iOS Version v4.0.7.
Requires iOS 9.0 or later. Compatible with iPhone 5s, iPhone SE, iPhone 6, iPhone 6 Plus, iPhone 6s, iPhone 6s Plus, iPhone 7, iPhone 7 Plus, iPad Air, iPad Air Wi-Fi + Cellular, iPad mini 2, iPad mini 2 Wi-Fi + Cellular, iPad Air 2, iPad Air 2 Wi-Fi + Cellular, iPad mini 3, iPad mini 3 Wi-Fi + Cellular, iPad mini 4 and iPad mini 4 Wi-Fi + Cellular. This app is optimized for iPhone 7, iPhone 7 Plus.
Android Version v4.0.6.
Android version 4.4.0 or later. Compatible with Samsung S7 edge, Samsung S7, Samsung S6, Samsung S5, Samsung NOTE4, Samsung NOTE3, Samsung tabs 705c, Ascend Mate 9, Ascend Mate 7, Huawei Honor 8, Huawei P8 Max, Vivo X7, Xiaomi 5, MI 3, Google Nexus 6p, Nexus 9, Google Nexus 7 II, LG V20, Sony Xperia Z3, Smartisan T1.