The notion of par 4's being the meat of a golf course, at least to me anyways, has to do with stringing shots together.
Par 3s can often be had with one solid shot, or even a lucky bounce, a worm burner that somehow rolls up and holds the green, etc...
Par 5's often allow for a shot to be wasted. Hit a decent drive, top one 30 yds, hit a good third and get up and down for par.
These types of holes have more room for error....and can impact your score with one swipe of the club.
But the par 4 rarely allows for such antics. You must hit a decent drive and you must get up near the green to have any chance of par, this side of a miraculous shot.
There is little opportunity for wasting a shot, and one perfect shot....one great swipe of the club, is not going to ensure a par.
Hit a great drive, well, you still have to hit a solid approach shot.
Hit a great approach shot, well, you still needed to hit a solid drive to get yourself in that position.
The par 4's require good shots being strung together, whereas I don't think the same demands are generally required on par 3s and 5s.