Not to state the obvious but your iMac will not be able to display at 4K, but it should play the files just fine when converted to 1080p. I recently took my
Mavic 2 Pro to a friends with an older iMac and I had to convert the files to play them without very bad stuttering problems. I used a few different players too. It just could not play the files at 4K well at all. Having said that it did play the downscaled ones perfectly and they looked great. We used a program called Handbrake to convert them on his iMac.
Edit: Its around a gig per minute (roughly)