JSFiddle - React, Tailwind, and code Playground

HTML

<form id="formId" action="mypage.php" method="post" target="_blank" />
<input type="hidden" name="turnaround" value="20">
<table>
    <tr>
        <td>Platform</td>
        <td>
            <select required id="cu" name="plat" style="min-width: 120px;">
                <option value="1">1</option>
                <option value="2">2 (Desktop Only)</option>
                <option value="3">3 (Mobile Only)</option>
                <option value="4">4 (Wildcard)</option>
                <option value="5">Catch-All</option>        
            </select>
        </td>
    </tr>
    <tr>
        <td colspan="2">&nbsp;</td>
    </tr>
</table>
<div><input type="submit" value="Select Platform" /></div>
</form>

JavaScript

$("#cu").on("change",function(){
    var a = $(this).val();
    if(a == "4"){
         $("#formId").removeAttr("target")
    }else{
    $("#formId").attr("target","_blank")
    }
    
})