<style>
	.next div {
		display: none;
	}
	.vis {
		display: block !important
	}
</style>

<script>
	function Selected(sel) {
		var next = sel.nextSibling.nextSibling; //nextElementSibling;
		var elems = next.getElementsByClassName('vis');//сделать кроссбраузерной
		if (elems.length != 0) {
			elems[0].className = '';
		}
		if (sel.options[0].value == 0) {
			if (sel.selectedIndex != 0) {
				next.children[sel.selectedIndex - 1].className = 'vis';
			}
		} else {
			next.children[sel.selectedIndex].className = 'vis';
		}
	}
</script>

<form> 

<select name="layer" aria-required="true" onChange="Selected(this)">
	<option value="0" selected="selected">no layer</option>
	<option value="1">layer 1</option>
	<option value="2">layer 2</option>
	<option value="3">layer 3</option>
	<option value="4">layer 4</option>
</select>

<div class="next">
	<div id='Label1'><span class="note">layer 1</div>
	<div id='Label2'><span class="note">layer 2</div>
	<div id='Label3'><span class="note">layer 3</div>
	<div id='Label4'><span class="note">layer 4</div>
</div>

<select name="layer" aria-required="true" onChange="Selected(this)">
	<option value="1">layer 1</option>
	<option value="2">layer 2</option>
	<option value="3">layer 3</option>
	<option value="4">layer 4</option>
</select>

<div class="next">
	<div id='Label1' class="vis"><span class="note">layer 1</div>
	<div id='Label2'><span class="note">layer 2</div>
	<div id='Label3'><span class="note">layer 3</div>
	<div id='Label4'><span class="note">layer 4</div>
</div>




</form>