<input
type="text"
id="kucukInput"
minlength="4"
maxlength="8"
size="20"
/>

<textarea id="buyukInput" rows="5" cols="33">
        </textarea>

let kucukInput = document.querySelector('#kucukinput);
let buyukInput = document.querySelector('#buyukinput);
kucukInput.addEventListener('keyup', function () {
  buyukInput.value = kucukInput.value;
});
buyukInput.addEventListener('keyup', function () {
  kucukInput.value = buyukInput.value;
});