Checkbox con funzionalità radiobutton

[Vindav]

<input id="check1" type="checkbox" disabled="disabled" checked="checked" onclick="functionTest(this)"/>
<input id="check2" type="checkbox" onclick="functionTest(this)"/>
<input id="check3" type="checkbox" onclick="functionTest(this)"/>
<input id="check4" type="checkbox" onclick="functionTest(this)"/>

function functionTest(el) {
    if(el.id != "check1"){
        document.getElementById('check1').checked= false;
        document.getElementById('check1').disabled = false;
    }else{
        el.disabled = true;
        document.getElementById('check2').checked = false;
        document.getElementById('check3').checked = false;
        document.getElementById('check4').checked = false;
    }
}