Well, you can try, but I doubt it will work. The old DJI GO 4 app dates from before there was a Mavic Air, i.e. it's not in the opening menu. Consequently, it will not recognize the presence of the birdie, not connect to it, and not modify the transmission parameters. :-(

If this hack worked with the Mavic Air I'd order one today. I'm perfectly happy with the range of the (hacked) Spark already and it stands to reason the Air would perform even better.

And yes, the hacked app is only available for Android, but once run, it works just fine with an up-to-date stock app on iPhone.

Here is the German forum where they're discussing it:
http://drohnen-freunde.de/forum/forum/index.php?thread/1130-aktiviere-den-fcc-mode-am-spark-bessere-%C3%BCbertragung-und-reichweite/&pageNo=4

ENGLISH READERS: The video is in English, so go ahead and check out the forum anyway.

The video shows how it works. If you read the thread, you'll find some people couldn't get it to work. For me, it works right out of the box. However, you can't do damage to the bird, so there's no harm in trying.

As to a range test (when this detestable weather goes away), don't worry about losing the birdie. I know, it's uncomfortable when you lose sight and suddenly the video signal breaks off, too. Scaaaary. Just make sure your compass is calibrated properly, there are PLENTY of satellites receivable AND YOUR HOMEPOINT IS ACTUALLY SET!

People lose drones because they're too lazy to do a proper preflight check. Don't be like that. I calibrate my compass at every new location. Takes a minute and gives piece of mind.

P.S.: The "bloody cold" greatly affects the batteries. Don't count on the telemetry for flight time; voltage could drop suddenly and the bird won't make it back and lands in the sticks.