I haven't tried yet but a 4K frame grab from the Mavic Air video should be equivalent to taking a still photo correct?

No, take a look at the link. The photo resolution is noted here: https://www.dji.com/mavic-air/info

Photo Resolution:
4:3: 4056×3040
16:9: 4056×2280

Video Resolution:
3840x2160p

Well, not so much: A 4K frame grab is only 8 mega pixels compared to a 12MP 4:3 still photo (or 9.2MP 16:9 still shot). Compression is also much higher/more lossy compared to a JPG still.

On top of that, if you want a smooth video you have to stop down (via ND filters) which gives you blurred individual frames. If you want good frame grabs (which is quite possible with MA) then you end up with jerky videos. So it is a compromise one way or the other...

Hi Diesel, as David mentioned, the resolution specs for the video and the still photos are different, please check it from DJI website, at the same time, you are recommended to read the user manual more than once before you fly the drone. Hope you enjoy the flight, thanks!

As others have quite rightly explained, the resulting image will be worse than that taken as a still.
However, the important thing is how you intend to view the image.
If you are printing A3, then at a normal viewing distance normal people will find it acceptable.
It wont make any difference if viewed on a phone or tablet screen, even a 1080p res monitor.
I used frame grabs for tourist information boards outside landmark buildings. they are large and clear for purpose.
Does that help you decide.