Javascript RegExp should check four-digit input for plausibility

I want to program a JavaScript code which checks the input into an input field if it is a double two-digit HEX value. The input must contain exactly 4 digits and letters between a to f and/or 0 to 9. The following code leads to an error with a four digit input.

function check() {
var input = (document.getElementById("textField").value);


if (input.length == 4 && input.match([A-Fa-f0-9])) { alert("the input matches the requirements");}
          else {alert("input is invalid");}
}

<input type="text" id="textField">
<button type="button" onclick="check()">check!</button>