JSFiddle - React, Tailwind, and code Playground

HTML

<form>
    <input class="input" onkeydown="isEmpty()" onkeyup="isEmpty()">
    <input class="input" onkeydown="isEmpty()" onkeyup="isEmpty()">
    <input class="input" onkeydown="isEmpty()" onkeyup="isEmpty()">
    <input class="input" onkeydown="isEmpty()" onkeyup="isEmpty()">
        <button id="submit" disabled>Enviar</button>
</form>

JavaScript

function isEmpty() {
    var submit = document.getElementById("submit");
    var inputs = document.getElementsByClassName("input");
    var empty =  Array.prototype.filter.call(inputs, function (input) {
        return input.value.trim() == "";
    });
    submit.disabled = !!empty.length;
}