I have a table rule that if you roll a nat 1 in combat, you roll a D4.

If you roll a 1 again - you accidentally damage yourself, or an ally.

If you roll a 2 - your weapon gets jammed and you are incapacitated until the end of your next turn (unless you have another weapon to use)

If you roll a 3 or a 4 - nothing happens.

I proposed this rule to my players as a way to make nat 1s feel more like a critical failure, with negative effects - just as a critical success gives you double damage.

The chances of rolling a nat 1, and then a 1 or a 2 are 1.25%, which I think is reasonable.

It's always been fine, but today my players rolled 7 nat 1s in 2 hours of playing (ouch). After the game, one of my players said that the D4 rule is unfair and they would never do that to their players. They've not brought this up before, and I think they are just frustrated, but I am fairly new to DM'ing (a year and a half ISH) and I'm always worried I'm not doing well.

Do you guys think it is unfair?

I just don't really think critical failures add anything to the game. You've already missed. It already feels kinda bad. Why do you need to make it worse?

Yes. I learned this lesson early on, Nat 1's already feel bad. Stay away from modifying by adding extra bad effects.

TLDR, it's definitely unfair

If everybody agrees it’s fine. I typically warn against any mechanic that takes a player’s turn away, stuns, sleeps, etc. it can easily be 20-30 minutes between turns at some tables so you don’t want to have arbitrary incapacitation.

Fumbles are not fun at all

Critical fumbles haven't been a part of the game for a long time for a reason. Don't do this

Doesn't sound fun at all lol

Adding rules that make combat less fun for non casters sounds like a great idea.

How does my hand get jammed when casting a spell? Unless this is only for martials in which case yeah treating them differently is weird

That aside, fumble charts do exist, you could take some inspiration there - but I would make it fair and also apply it to the enemies at least. And if your players don't like it, maybe just don't do it

In my experience, players rarely enjoy critical fumble. Having it to be your turn and rolling a 1 and having it be an automatic failure is enough of a punishment.

And when you factor in how many attacks certain classes get per round, now you are making it so that players can hurt themselves more often even though they are becoming better and stronger.

Some players like it, but honestly DMs who have critical failure/fumbles are DMs that have a bad understanding of their role in the game.

Yeah, this sucks. Nobody actually likes crit fumbles and it disproportionately punishes already underpowered martial classes. Don’t do this. Look up advice about crit fumbles and the feedback will be overwhelmingly negative.

Crit fumble rules can fuck off back to the white room where they come from.

I dont like thise kinds of rules as a crit one sucks anyway. I dont think it adds to the fun of the table. In this case it also adds more dice and thus more time. However, if you and your players like it it doesnt seem too harsh, I just dont see what that it brings anything useful.

It is possible to argue that it also hurts martials more and is thus beating down characters who are often still the weakest, but this isnt the worst thing.

I wouldnt do something like this myself but IT seems everyone agreed to the rule and thats all there is to say.

I have further questions: does this rule effect casters or enemies in the same way?

I would caution new DMs to avoid adding homebrew rules until they have a good feel for the regular mechanics. Fumble rules are one of those homebrews that are frequently added by DMs that don't know better.

Is this rule adding to the fun for you and your players? Sounds to me like the answer is No, so I would probably advise you to drop it.