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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
  <label for="op1"><input type="checkbox" value="1" name="op1[]" id="op1"  />Opção 01</label></p>
  <label for="op2"><input type="checkbox" name="op1[]" value="2" id="op2" />Opção 01</label></p>
  <label for="op3"><input type="checkbox" name="op1[]"  value="3" id="op3" />Opção 01</label></p>
  <label for="op4"><input type="checkbox" name="op1[]" value="4" id="op4" />Opção 01</label></p>
  <input type="button" id="btn" value="verificar">
</body>
</html>
 
var opts = document.getElementsByName("op1[]");
​
var isChecked = function() {
  for (var i = 0; i < 4; i++) {
    if (opts[i].checked) return true;  break;
       
  }  
};
​
var btn = document.getElementById("btn");
btn.onclick = function () {
  if (!isChecked()) {
    alert("Selecione uma opção");
  }
};
​
Output

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anonymouspro
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