<script>
function checkOthers(cb) {
    cb.form.elements['checkbox1'].checked = cb.checked;
    cb.form.elements['checkbox2'].checked = cb.checked;
    cb.form.elements['checkbox3'].checked = cb.checked;
    cb.form.elements['checkbox4'].checked = cb.checked;
}
function checkIfAllChecked(cb) {
    cb.form.elements['checkbox5'].checked = cb.form.elements['checkbox1'].checked && cb.form.elements['checkbox2'].checked && cb.form.elements['checkbox3'].checked && cb.form.elements['checkbox4'].checked;
}
</script>
<label>checkbox 1 <input type="checkbox" id="checkbox1" name="checkbox1" onclick="checkIfAllChecked(this);" /></label>
<label>checkbox 2 <input type="checkbox" id="checkbox2" name="checkbox2" onclick="checkIfAllChecked(this);" /></label>
<label>checkbox 3 <input type="checkbox" id="checkbox3" name="checkbox3" onclick="checkIfAllChecked(this);" /></label>
<label>checkbox 4 <input type="checkbox" id="checkbox4" name="checkbox4" onclick="checkIfAllChecked(this);" /></label>
<label>checkbox 5 <input type="checkbox" id="checkbox5" name="checkbox5" onclick="checkOthers(this);" /></label>