<div>
    <a href='#' id='link1' onclick='toggle(this)'>link 1</a>
    <a href='#' id='link2' onclick='toggle(this)'>link 2</a>
    <a href='#' id='link3' onclick='toggle(this)'>link 3</a>
    <a href='#' id='link4' style='display:none' onclick='toggle(this)'>link 4</a>
</div>

var disp = false;

function toggle(el) {
    if (disp) {
        if (el.id == 'link4') {
            document.getElementById('link4').style.display='none';
            disp = false;
        }
    } else {
        if ((el.id == 'link1') || (el.id == 'link2') || (el.id == 'link3')) {
            document.getElementById('link4').style.display='block';
            disp = true;
        }
    }
}

var disp = false;

function toggle(el) {
    if (disp) {
        if (el.id == 'link4') {
            $('#link4').fadeOut();
            disp = false;
        }
    } else {
        if ((el.id == 'link1') || (el.id == 'link2') || (el.id == 'link3')) {
            $('#link4').fadeIn();
            disp = true;
        }
    }
}

var 3rdLink = $("#3rdLinkId"),
    4thLink = $("#4thLinkId"); //just good practice
3rdLink.on('click', function(event){
    event.preventDefault();  //just in case it's a hyperlink
    4thLink.css('visibility', 'visible');
)};

<a href="#" class="show" onclick="show4()" >link 1</a>
<a href="#" class="show" onclick="show4()" >link 2</a>
<a href="#" class="show" onclick="show4()" >link 3</a>
<a href="#" id="link4" class="hide" onclick="hide4()" >link 4</a>

<style>
.hide {display: none;}
.show {display: block;}
</style>
<script>
function show4()
{
    document.getElementById('link4').style.display = "block";
}
function hide4()
{
        document.getElementById('link4').style.display = "none";
}
</script>

<div id="link-container">
    <a href="#">link1</a>
    <a href="#">link2</a>
    <a href="#">link3</a>
    <a href="#">link4</a>
</div>

#link-container a:last-child{display:none}

$(function(){
    $('#link-container').on('click', 'a',function(e){
        var $t = $(this),
            $last = $t.siblings('a').addBack().last();
        if($last[0] != this){
           $last.toggle();
        }
    });
});