So from what i understand, when you use the RC the Wifi connection is just between your phone and the RC. This is the Wifi connection that is "unstable". The Connection between the RC and the Aircraft (also being Wifi i guess) is quite stable.
It seems that some people (me included) lose wifi connection during flight and the picture starts to stutter or even goes black.

So what i don't get. How can the Wifi connection between RC and phone be problematic? They are right close by eachother. The RC acts as Hotspot and the phone connects... how can that be unstable?
And why can an OTG cable be so much more stable, if its just the connection between phone and RC??

It's problematic because of everything going on around the phone. There are connection issues even being close together. DJI could make it better but because of price,  I'm sure, they don't. The wifi connection works very well most of the time but it's doomed to disconnect and very often. Some pilots are not having problems with it. I did and now run an OTG cable between the 2. I hear Andoid phones are awful compared to Apple.

im not having lots of problems, but i have noticed it disconnecting when i get a little further. i havent gone much further than 200m though and have already had disconnections of laggings!

so you say the interference is comming from around the phone, but those are there all the time, why is it only when the drone gets far enough, that there are disconnections or lags?
this would mean the distance does make a difference.

I haven't experienced the issues you described, but keep in mind that due to bandwidth requirements of transmitting data, (in this case 720P) that when there is limited signal, the AC and RC will prioritize the control and telemetry (altitude, speed, distance, location) signal over the video signal in order to maintain flight and control over the aircraft during signal degredation.  so what you may be seeing as signal loss between the RC and phone, might just be some slight interference between the spark and remote.  If you're seeing the video drop but not the telemetry, i would bet that is what is happening.  

Nikp, the RC and mobile connected via WiFi which is easily disturbed by the external factors, such as environment. While connecting via OTG has not been officially tested  and the stability can not be ensured, so we do not suggest to use this feature for now. Appreciate your understanding.

Nikp, the RC and mobile connected via WiFi which is easily disturbed by the external factors, such as environment. While connecting via OTG has not been officially tested  and the stability can not be ensured, so we do not suggest to use this feature for now. Appreciate your understanding.

What I can tell is the Wifi icon on Dji Go 4 show combined signal strength of Spark to RC and Mobile device. What I learned is if I use iPad Pro 10 inches, though I put the iPad down on the ground and sit on a chair controlling Spark using RC, the singal strength showing on iPad is very good. But if I change mobile to iPhone 6s plus, put iPhone at the same place, the signal strength drop to lowest. Now mount iPhone to mobile holder at RC the signal strength is good. The test is base on the location of Spark are the same. The reason is the antenna of iPad pro has more gain than iPhone and the wifi on RC for mobile connection is designed to be low power such that it can communicate well with the mobile at the holder rather than the mobile of someoneelse.

Is it normal to lose signal around 200m when there are trees? not woods, but many trees..
I flew again today, i had good control, but i did lose the signal a few times for a short period of time. As i said, i've only been about 200m away from the drone, but its not clear sight. there are trees inbetween.

Yes. Everything can decrease signal strength. First is the distance. Second is any object in the line between rc and Spark. Third the gas, vapor, dust like fog. Fourth is the radio frequency from nearby source. Fifth is the angle between the RC antenna and Spark, the antenna pointing to Spark is the worst while the side of antenna pointing to Spark is best. Try flying higher at longer distance with the side of the antenna face Spark. Make sure that Spark can see you always when you turn its camera to you. Stay away from radio station or mobile tower.