As we know,openHD can be use on Pi3B+ and Pi4B.But I have a question，what is different between Pi4B and Pi3B+ in using（expect hardware improvement）.Is Pi4B has big improvement？
Hi
When switching from Pi3 to Pi4 as a Groundstation think the biggest improvement of Pi4 is the better USB part.
It allows to connect many more and faster connections like
4+ ground cards (while the Pi3b is maxed out with 2-3)
Usb3 Stick can save the recorded video much faster. Seconds vs minutes.
Bigger RAM allows to record much longer videos.
2 HDMI ports are nice too, but micro.
Also I think I have a better Wifi connection to the Pi4‘s hotspot, but no data to support this lol hard to measure.
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OK，thanks for your answer.I will buy Pi4B 2g RAM as ground PI and Pi3A+ as air Pi.
Sometimes I think we’re lucky to get even one wifi card working on the Pi3 series 
Hi Stephen.
Are you recommending Pi4B for airPi and Grd station?
For a beginner would you recommend not to bother with Pi3?
My fixed wing drone can easily carry Pi4 or Pi3. Am not bothered with PiZero.
Yea it’s a good idea to go for the pi4 even if just because the USB connection is significantly better (not just faster, we don’t care much about that, but more stable and reliable).