[Царь Леонид]

Царь Леонид,
а без цикла сможешь?

<h2>Доставка</h2>
<form action="">  
<input type="radio" id="deliveri_1" name="deliveri" value="1">
<input type="radio" id="deliveri_2" name="deliveri" value="2">
<input type="radio" id="deliveri_3" name="deliveri" value="3"> <!-- Если выбрали этот то... -->
<input type="radio" id="deliveri_4" name="deliveri" value="4">
</form>

<h2>Оплата</h2>
<input type="radio" id="payment_1" value="1">
<input type="radio" id="payment_2" value="2"> <!-- ...то этот станет недоступен -->
<input type="radio" id="payment_3" value="3">
<input type="radio" id="payment_4" value="4">
<script>
var selected = document.querySelectorAll('input'),
 makeDisable = document.getElementById('payment_2');

selected.forEach(function(item){
item.addEventListener('change', function(event){
  switch(event.target.id){
    case 'deliveri_3':
      makeDisable.disabled=true
      makeDisable.setAttribute('class', 'random')
      return null; 
    case 'deliveri_4':
      makeDisable.setAttribute('class', 'random')
      makeDisable.disabled=false
      return null;
    default:
      makeDisable.removeAttribute('class', 'random');
      makeDisable.disabled=false
      return null;
  } 
}) 
})
</script>