How stupid should a low INT player act?

How would you handle a player with a very low stat that makes him a liability to the party''s goals? 

I'm assisting the DM on a quick scenario to introduce two new players (high schoolers) to the game. One player has 4 INT and he really takes to the role, doing "stupid" things. I'm accompanying the party as a wizard to show them how the game works but I'm spending more time managing low INT guy. Thank you. 

"Intelligence" is a complex subject, and really all of the mental stats cover some of what we consider to be intelligence.

The Int stat, despite its name, is much more about knowledge than anything else.

A low-int character ought to know very little, and not be good at reaching reasoned conclusions. 

However, I get the impression that what the player in question is doing is doing disruptive things, with the excuse of "my character is dumb".

If that's the case, you need to have a chat with them outside of play, because the problem is not them playing an unintelligent character, but them being disruptive to other people's fun. (This includes you.)

It's entirely possible to play an unintelligent character without doing that. It's possible they're just not thinking about what intelligence means in-game, and are playing to a bad stereotype.

But it's also possible that they're enjoying making a mess of things. Your conversation will determine what, if any, adjustments will be made. (Remember: "it's what my character would do" is not a get-out-of-jail-free card for bad in-game behavior. The player still chooses to make the character, and people need to make characters who can function with a group and handle the day-to-day life of adventuring.)

Also, the standard methods of character creation in 5e don't allow for a stat lower than 8. Having presumably decided to use a method that allows for extremely low stats, you are going to have to cope with it to a degree.

Low intelligence means limited knowledge or mathmatical ability, not foolishness.  A low intelligence character won't stick their hand in a fire just to see what it is or be highly gullible, for example, those are traits of a low wisdom character.

I think it is worth remembering there are two other mental skills at play - Charisma and Wisdom.  A character with low intelligence, but decent wisdom, for example, might not be very book smart, but they are going to be worldly and have a basic knowledge of reading other people and situations.  A character with low intelligence, but high charisma, might likewise be bad at books, but have a decent understanding of how to interact with others in the world and how to carry themselves in a way that others find acceptable.  

So, you could be someone like Forrest Gump (bad at all the mental skills, but manages to do so in a lovable and endearing way).  Gilderoy Lockhart, in Harry Potter, constantly displays low intelligence and wisdom, but his charisma is off the charts and he made a very successful career on charisma alone.  Samwise Gamgee, in LotR, has very high wisdom and, while not low in intelligence or charisma, is not a shining star in either, but he manages to use his heart and general strength of character to get through dangerous situations.  

Lots of different ways to play this right, and the person should look at their entire character.  However, it is worth noting there are ways of playing this VERY wrong - especially at a character with that low of an intelligence stat, you could veer into parody of characters with mental disorders, and that is a really bad place to be that could very easily offend others at the table. 

I would say that the numbers on a sheet that are just for rolls, and a character doesn't have to be actually of limited intellect. Sometimes it is fun to play the numbers, but it isn't required. I played a barbarian of a 8 INT, but that only mattered when I was asked for an Intelligence Check of some kind, and you can push the RP for a good long while with average intelligence behaviors and reasoning skills before a Check is even asked for.

So how "stupid" should a low INT player act? The answer, in my mind, is as stupid as is fun for everyone at the table. 

I’d add that “doing stupid things” is usually more a low wis character than a low int. Bad wis is when you make bad choices. 
I didn’t come up with these, but if it helps, when you feel water drops on your arm, int tells you it’s raining, wis tells you to go inside. Or my other favorite, int tells you a tomato is a fruit, wis tells you not to put it in a fruit salad.

All that said, the line can be really unclear.

I'd agree with jl8e. There's not a lot of description, but the impression I have is of a player who is using low INT as an excuse to be disruptive to the group for his own amusement. That's not a good fit for a collaborative game.

Agreed with what others have said. 

I also think the most important part is to remember that, at the end of the day, this is a hobby and a game. If someone is making the game less fun for everyone else (including you, as the DM), then they should stop it. Even if they have a perfectly rational in-universe explanation.

Unless a teenager has a sibling or close relative that is mentally challenged, teenagers might not have the maturity to roleplay a low intelligent PC.  The DM/PC should probably waive any score below 8 and have an intelligence ability no lower than 8.  

My experience with mentally challenged is they are more inclined to not know and/or remember something.  But they do not wander, usually staying in one place.  They would have a negative stealth skill as volume in talking is never a whisper.  They may be able to be silent, but any curiosity they feel, is spoken with questions several decibels about a whisper. 

An imperfect comparison of INT and normal real world is to consider the INT score to be like an IQ.  A 10 = 100 IQ which is normal.  An 8 = 80 which would be considered trainable classification for a mentally challenged person.  Trainable allows them to have a job and be able to conduct menial jobs in the service field. (e.g. at a hotel, working as a housekeeper)