Hi.

I recently purchased a DJI Mavic Air.
  Once I have transferred the 4k and 2k videos to the hard disk of the imac (2009), these are played jerkily and with pauses. In MP4 and MOV format.
  But when playing them on the laptop (2012) with windows 10, they are perfectly reproduced.

  Any solution?

  Thank you.

Your 2009 computer is too old to process high resolutions videos.

Consider upgrading your hardware.

I too have a 2009 iMac. It’ll play 3k just but the hardware just can’t cope with 4K. You need a newer machine, record in a lower resolution or use proxies for editing to work.

Ok, thank you both.
For now I will try to continue with my imac i5 of 2009 recording the videos at 1080p.
The strange thing is that my 2012 laptop (lenovo yoga 2 11 ") can play 4k videos without problems.