diff --git a/simplified drill-down v4 b/simplified drill-down v4
new file mode 100644
index 0000000..6b0888c
--- /dev/null
+++ b/simplified drill-down v4	
@@ -0,0 +1,178 @@
+<!DOCTYPE html>
+<html lang="en">
+  <head>
+    <meta charset="utf-8">
+	<script src="http://d3js.org/d3.v4.min.js"></script>
+	<script src="http://d3js.org/topojson.v1.min.js"></script>
+	<script
+		src="https://code.jquery.com/jquery-3.3.1.min.js"
+		integrity="sha256-FgpCb/KJQlLNfOu91ta32o/NMZxltwRo8QtmkMRdAu8="
+		crossorigin="anonymous"></script>		
+    <style>
+    #map {
+      background-color: #fff;
+      border: 1px solid #ccc;
+    }
+    .background {
+      fill: none;
+      pointer-events: all;
+    }
+    #countries, #states {
+      fill: #cde;
+      stroke: #fff;
+      stroke-linejoin: round;
+      stroke-linecap: round;
+    }
+    #countries .active, #states .active {
+      fill: #89a;
+    }
+    #cities {
+      stroke-width: 0;
+    }
+    .city {
+      fill: #345;
+      stroke: #fff;
+    }
+    pre.prettyprint {
+      border: 1px solid #ccc;
+      margin-bottom: 0;
+      padding: 9.5px;
+    }
+    </style>
+  </head>
+  <body>
+    <div id="map"></div>
+
+    <script src="//d3js.org/d3.v4.min.js"></script>
+    <script src="//d3js.org/topojson.v1.min.js"></script>
+    <script>
+    var m_width = $("#map").width(),
+        width = 938,
+        height = 500,
+        country,
+        state;
+
+    var projection = d3.geoMercator()
+        .scale(150)
+        .translate([width / 2, height / 1.5]);
+
+    var path = d3.geoPath()
+        .projection(projection);
+
+    var svg = d3.select("#map").append("svg")
+        .attr("preserveAspectRatio", "xMidYMid")
+        .attr("viewBox", "0 0 " + width + " " + height)
+        .attr("width", m_width)
+        .attr("height", m_width * height / width);
+
+	// we create a rect area with an event listener associated to it	
+    svg.append("rect")
+        .attr("class", "background")
+        .attr("width", width)
+        .attr("height", height)
+        .on("click", country_clicked);
+		
+
+    var g = svg.append("g");
+
+	// the first map to show is the countries.topo.json
+	// the click event is also associated to each country
+    d3.json("json/countries.topo.json", function(error, us) {
+      g.append("g")
+        .attr("id", "countries")
+        .selectAll("path")
+        .data(topojson.feature(us, us.objects.countries).features)
+        .enter()
+        .append("path")
+        .attr("id", function(d) { return d.id; })
+        .attr("d", path)
+        .on("click", country_clicked);
+    });
+
+    function zoom(xyz) {
+      g.transition()
+        .duration(750)
+		//translate(x,y), scale(z)
+        .attr("transform", "translate(" + projection.translate() + ")scale(" + xyz[2] + ")translate(-" + xyz[0] + ",-" + xyz[1] + ")") 
+        .selectAll(["#countries", "#states", "#cities"])
+		//scale the path width
+        .style("stroke-width", 1.0 / xyz[2] + "px")
+        .selectAll(".city")
+		//scale the point radius
+        .attr("d", path.pointRadius(20.0 / xyz[2]));
+    }
+
+	//get width, height and scale based on the path of the selected country
+    function get_xyz(d) {  
+      var bounds = path.bounds(d);
+      var w_scale = (bounds[1][0] - bounds[0][0]) / width;
+      var h_scale = (bounds[1][1] - bounds[0][1]) / height;
+      var z = .96 / Math.max(w_scale, h_scale);
+      var x = (bounds[1][0] + bounds[0][0]) / 2;
+      var y = (bounds[1][1] + bounds[0][1]) / 2 + (height / z / 6);
+      return [x, y, z];
+    }
+
+	// the function associated to the click on the countries map
+    function country_clicked(d) {
+	  //we clean the graph, 
+      g.selectAll(["#states"]).remove();
+  	  g.selectAll("#USA").style('display', null);
+      state = null;
+
+
+	  //the call picks the data associated to the path
+      if (d && country !== d) {
+        var xyz = get_xyz(d);
+        country = d;
+
+        if (d.id  == 'USA') {
+		  //in this case we do semantic-drill-down zooming
+          d3.json("json/states_" + d.id.toLowerCase() + ".topo.json", function(error, us) { //we load the specific json
+			//we create the new map
+            g.append("g")
+              .attr("id", "states")
+              .selectAll("path")
+              .data(topojson.feature(us, us.objects.states).features)
+              .enter()
+              .append("path")
+              .attr("id", function(d) { return d.id; })
+              .attr("class", "active")
+              .attr("d", path)
+			  .on("click",function(){
+				//remove the states map
+				g.selectAll("#states").remove();
+				//recover the path of USA
+				g.selectAll("#USA").style('display', null);
+				//revert the zooming
+				var xyz = [width / 2, height / 1.5, 1];
+				country = null;
+				zoom(xyz);
+			  });
+			
+			//apply the new zoom	
+            zoom(xyz);
+			// hide the original country path	
+            g.selectAll("#" + d.id).style('display', 'none');
+          });      
+        } else {
+		  //we only do semantic zoomoing
+          zoom(xyz);
+        }
+      } else {
+		//we revert the zooming
+        var xyz = [width / 2, height / 1.5, 1];
+        country = null;
+        zoom(xyz);
+      }
+    }
+
+
+    $(window).resize(function() {
+      var w = $("#map").width();
+      svg.attr("width", w);
+      svg.attr("height", w * height / width);
+    });
+    </script>
+  </body>
+</html>
