Si me selleccionan un radio no se pueda excribir en el text

[Seppo]

Código HTML:

<form name="form1" method="post" action="">
  <input name="radiobutton" type="radio" value="radiobutton" onClick="document.form1.text.disabled=true">1<br>
  <input name="radiobutton" type="radio" value="radiobutton" onClick="document.form1.text.disabled=true">2<br>
  <input name="radiobutton" type="radio" value="radiobutton" onClick="document.form1.text.disabled=true">3<br>
  <input name="radiobutton" type="radio" value="radiobutton" onClick="document.form1.text.disabled=false">4
  <input type="text"name="text" disabled >
</form>