I brought a cheap mobile phone, Sugar Y12 (3G ram + 32Gb storage. Android 7.0, USD110). To test out the latest DJI Go 4 Apps 4.2.21 and connection to Spark. I found that Sugar Y12 using wifi connection, video transmission was laggy and not smooth. I changed to OTG USB, the video transmssion was smoother than wifi connecxtion. It was acceptable but not the best.

I uninstalled DJI Go 4 Apps 4.2.21 and loaded 4.2.12 onto the Sugar Y12. Using wifi and OTG USB connections, both options were smooth and faultless. That means 4.2.12 consumes much less phone resources than 4.2.21.

I will use the Sugar Y12 exclusively for flying Spark.

Note: AC firmware .701, RC firmware .600. Sugar Y12 was on airplane mode with GPS on. I encountered "resource file error" durng the installation of DJI Go 4 4.2.12. I fixed the problem by first installing DJI GO 4 4.2.21 and flew the Spark. Took the bird down and uninstalled 4.2.21. Re-booted the Sugar Y12 and installed 4.2.12. 4.2.12 loaded successfully.

If using wifi connection beween the Sugar Y12 and RC, only 5.8G wifi channels between RC and AC can give out smooth video and stable connection. If usiing OTG USB, both 2.4G and 5.8G wifi connection between RC and AC, can give out smooth and stable video connection. Video quality is better on 5.8G between RC and AC. By default, DJI Go 4 App uses wifi 2.4G to make connection with smartphone, therefore, cannot use 2.4G wifi to connect between RC and AC. This is a design limitation.

DJI Go 4 app should ban user from chosing 2.4G wifi (custom) connection between RC and AC, and only opens 5.8G channels for selection, if user uses wifi connection between smartphone and RC. The working combinations can be explained in the following diagram:
Smartphone ---wifi--> Remote Control --5.8G--> Spark
Smartphone --USB OTG--> Remote Control --2.4G/5.8G--> Spark

I brought a cheap mobile phone, Sugar Y12 (3G ram + 32Gb storage. Android 7.0, USD110). To test out the latest DJI Go 4 Apps 4.2.21 and connection to Spark. I found that Sugar Y12 using wifi connection, video transmission was laggy and not smooth. I changed to OTG USB, the video transmssion was smoother than wifi connecxtion. It was acceptable but not the best.

I uninstalled DJI Go 4 Apps 4.2.21 and loaded 4.2.12 onto the Sugar Y12. Using wifi and OTG USB connections, both options were smooth and faultless. That means 4.2.12 consumes much less phone resources than 4.2.21.

I will use the Sugar Y12 exclusively for flying Spark.

Note: AC firmware .701, RC firmware .600. Sugar Y12 was on airplane mode with GPS on. I encountered "resource file error" durng the installation of DJI Go 4 4.2.12. I fixed the problem by first installing DJI GO 4 4.2.21 and flew the Spark. Took the bird down and uninstalled 4.2.21. Re-booted the Sugar Y12 and installed 4.2.12. 4.2.12 loaded successfully.

If using wifi connection beween the Sugar Y12 and RC, only 5.8G wifi channels between RC and AC can give out smooth video and stable connection. If usiing OTG USB, both 2.4G and 5.8G wifi connection between RC and AC, can give out smooth and stable video connection. Video quality is better on 5.8G between RC and AC. By default, DJI Go 4 App uses wifi 2.4G to make connection with smartphone, therefore, cannot use 2.4G wifi to connect between RC and AC. This is a design limitation.

DJI Go 4 app should ban user from chosing 2.4G wifi (custom) connection between RC and AC, and only opens 5.8G channels for selection, if user uses wifi connection between smartphone and RC. The working combinations can be explained in the following diagram:
Smartphone ---wifi--> Remote Control --5.8G--> Spark
Smartphone --USB OTG--> Remote Control --2.4G/5.8G--> Spark