I am sure he wanna used it for shitcoin, maybe those shitcoin with lots of zeros where 4$ would be giving him about 1b of the token. Usually it could be those meme coin that is over hyped on social media and allover. Exchange like binance wouldn't allow you to trade $4 there minimal is $10 but I don't know about that exchange you mentioned.
We are talking about futures trading and what we should care about is our leverage and position , we will not receive any tokens in futures trading .
That's quite a small amount and I'm not sure he can start trading with that small amount . But if possible this is a wise move as his goal could be to experience futures trading . If he is newbie and using minimum amount to try futures trading is the right decision instead of using large amount. Because as we all know, futures trading is very risky and is no different from gambling .
$4 is also money and may not be anything to many people here but it may be a significant amount to others . I don't see any problem with starting trading with $4 if the exchange allows using that amount for trading .