JSFiddle - React, Tailwind, and code Playground

HTML

<input id="fournumbers" name="fournumbers" type="text" value="">

JavaScript

var fournumbers = document.querySelector('#fournumbers');

fournumbers.addEventListener('keyup', blockNumbers, false);

function blockNumbers() {
    var value          = this.value;
    var last_character = value[value.length-1];
    var reg            = new RegExp('^[0-9]$');
    var character      = value.split('');
    var count          = 0;
    
    for(var i = 0; i < character.length; i++){
        if(reg.test(character[i])) {
            count++;
        }
    }
    
    if (count >= 4) {
        console.log('Ok, mas como bloquear a digitação de mais números?');
    }
}