There are legit par 4s and 5s in disc golf, but there aren't that many.
My home course has one, hole 10 at Hiestand (long pad to blue pin), but it isn't marked and I count it as a "par 3" for scoring purposes. If I get a 4 there though, I'm not too bummed about it, because I know that the rest of the field is going to card some 4s there too. I consider it a par 4, but don't really care because it's the number of throws that ultimately counts.
Determining par for courses can be done systematically but often is not. Token Creek and Cap Springs in my area have absolutely ridiculous course pars. They are laughably inflated, and if someone tells me they use course par at those courses, my opinion of their game changes (I can't help it).
This guy on Reddit yesterday asked if anybody had any suggestions for a replacement for his Predator, because that game took his game to the next level and he wanted to take it to another level yet again (the nextest level). He claimed to shoot on average -17 to -20 on his home course (Baraboo Lions) from the long tees. Then he said he shot -12 whenever he played the courses here in Madison. For reference: -2 at Hiestand longs to blue pins is about 1000 rated. I was going to ask this guy if he was trolling, but I don't think he was. I didn't ask: turning off the computer seemed the best response to that.
But this brings up the problem at hand here: communication. What is this guy talking about? What are his actual scores? Obviously, he's not that good a player, but how bad is he? We don't know, because he's going by some weird course par that somebody (maybe him, maybe someone else) made up.
Sometimes more casual players have asked a group of us what we shot and if we shoot all par 3s. This happened at Hiestand once, and I wanted to ask, what par do you go by? Do you determine par on the teepad? The par is then a kind of ad hoc thing, something you and your friends made up, but if you're going to communicate to the outside world, you have to have a baseline. A "par 3" has to be that baseline.
I'd love to see a tradition of accurately determining course pars that makes its way to good signage on the course. But that's not going to happen in disc golf, and I don't think it matters anyway. Counting throws works well enough.
But yeah, if you shoot in the high 60s at Iron Hill gold, you've scored some birdies. But that course is one of a handful that are legit gold level courses with a coherent sense of design that has par 4s and 5s as an intentional part of the design. 95% of courses have nothing like that kind of consistency of intent, so determining course par on these courses tends to be done on a whim.