D3: How you can use Tooltips in v4

<!DOCTYPE html>
<html lang="en">
	<head>
		<meta charset="utf-8">
		<title>D3: Tooltips in v4</title>
    <script src="https://d3js.org/d3.v4.min.js"></script>
		<style type="text/css">

rect:hover {
      fill: orange;
    }

#tooltip {
      position: absolute;
      width: 200px;
      height: auto;
      padding: 10px;
      background-color: white;
      -webkit-border-radius: 10px;
      -moz-border-radius: 10px;
      border-radius: 10px;
      -webkit-box-shadow: 4px 4px 10px rgba(0, 0, 0, 0.4);
      -moz-box-shadow: 4px 4px 10px rgba(0, 0, 0, 0.4);
      box-shadow: 4px 4px 10px rgba(0, 0, 0, 0.4);
      pointer-events: none;
    }

#tooltip.hidden {
      display: none;
    }

#tooltip p {
      margin: 0;
      font-family: Futura;
      font-size: 16px;
      line-height: 20px;
    }

</style>
</head>
<body>
  <div id="tooltip" class="hidden">
    <p><strong>Percentage Value</strong></p>
    <p><span id="value">100</span>%</p>
  </div>
		<script type="text/javascript">

//Width and height
			var width = 1200;
			var height  = 500;

var dataset = [ 5, 10, 13, 19, 21, 25, 22, 18, 15, 13,
							11, 12, 15, 20, 18, 17, 16, 18, 23, 25 ];

var xScale = d3.scaleBand()
							.domain(d3.range(0, dataset.length))
							.range([0, width], 0.05);

var yScale = d3.scaleLinear()
							.domain([0, d3.max(dataset)])
							.range([0, height]);

var svg = d3.select("body")
						.append("svg")
						.attr("width", width)
						.attr("height", height);

svg.selectAll("rect")
			   .data(dataset)
			   .enter()
			   .append("rect")
			   .attr("x", function(d, i) {
			   		return xScale(i);
			   })
			   .attr("y", function(d) {
			   		return height - yScale(d);
			   })
			   .attr("width", xScale.bandwidth())
			   .attr("height", function(d) {
			   		return yScale(d);
			   })
			   .attr("fill", function(d) {
					return "rgb(0, 0, " + (d * 10) + ")";
			   })
         .on("mouseover", function(d) {

var xPosition = parseFloat(d3.select(this).attr("x")) + xScale.bandwidth() / 2;
					var yPosition = parseFloat(d3.select(this).attr("y")) / 2 + height / 2;

d3.select("#tooltip")
						.style("left", xPosition + "px")
						.style("top", yPosition + "px")
						.select("#value")
						.text(d);

d3.select("#tooltip").classed("hidden", false);

})
			   .on("mouseout", function() {

d3.select("#tooltip").classed("hidden", true);

})
			   .on("click", function() {
			   		sortBars();
			   });

var sortOrder = false;

var sortBars = function() {

sortOrder = !sortOrder;

svg.selectAll("rect")
				   .sort(function(a, b) {
				   		if (sortOrder) {
					   		return d3.ascending(a, b);
				   		} else {
					   		return d3.descending(a, b);
				   		}
				   	})
				   .transition()
				   .delay(function(d, i) {
					   return i * 50;
				   })
				   .duration(1000)
				   .attr("x", function(d, i) {
				   		return xScale(i);
				   });

};

</script>
	</body>
</html>

https://d3js.org/d3.v4.min.js