Also worth noting, according to David Epstein who consulted biomechanics experts, a cinder track is about 1.5% slower than synthetic (assuming it is in good condition, more if worse).
Applying that 1.5% conversion factor to Ryun's 3:58.3 gives you 3:54.73.
So accounting for nothing else but the surface, his run was the equivalent of a solo 3:54.73 mile.
It's also worth noting, as you can see in the video above, he ran a negative split, with a very fast last lap. Run evenly, that time was worth even faster.
And perhaps the most impressive factor was that Jim did this from the front. If Jim had been in a fast race where he could have drafted off of a rabbit or faster runners for at least 3 laps and just focus on time, I think he would still have the record. His famous race against Snell was more about winning. It would probably be around 3:51.