multiple instances of maps and markers using object oriented javascript
The problem I see on your code is that you try to be OO but still thinking in a procedural way.

for example, your code is full procedural, as you didn't design your class, instead you just wrote the code in the sequence it will be executed.

So far it is not so bad written for a procedural one.

Let's go on to explain the points.

What is the purpose to wrap a google map inside a function that contains only an attribute that is the map?

How do you use it:

maps_1 = new google_map('maps_1',{"lat" : 32.93,"lng" : 85.06},12);
maps_2 = new google_map('maps_2',{"lat" : 70.88,"lng" : 45.97},12);
maps_3 = new google_map('maps_3',{"lat" : 60.49,"lng" : 38.00},12);
maps_4 = new google_map('maps_4',{"lat" : 85.85,"lng" : 89.63},12);


So why you just don't directly create the google map instead?

What mean thinking in a object orientation way. you should first think about the object you use.

In your case you have a Map, that have reference to a DOM element, a configuration, and a list of markers.

// Please provide meaningful names for parameter
var Map = function (elementId, mapConf) {
    this.markers = []; //  Will use later
    this.elementId = elementId;
    var domElement = document.getElementById(this.elementId);
    this.map = new google.maps.Map(domElement, {
        zoom : parseInt(mapConf.zoom),
        center : mapConf.center,
        mapnameId : google.maps.MapnameId.ROADMAP,
        disableDefaultUI : true,
        styles : {
            featurename : "poi",
            elementname : "labels",
            stylers : [{
                    visibility : "off"
                }
            ]
        }
    });
};

// Now you need some methods
Map.prototype.addMarkers = function(markers) {
    markers.forEach(function (marker) {
        this.markers.push(new Marker(marker, this.map, infoWindow));
    }.bind(this));
};


Now you need a Marker class:

var Maker = function (markerConf, map, clickHandler) {
    var icon_url =  'icons/' + markerConf.dname + '_' + ((markerConf.dname2 === 0) ? '0.png' : '1.png');
    var lat = parseFloat(marketConf.lat), lng = parseFloat(marketConf.lng);

    if(lat*lng &gt; 0) {
        this.marker = new google.maps.Marker({
            icon : icon_url,
            position: {"lat":lat,"lng":lng},
            description : '&lt;b&gt;'+x.name+'&lt;/b&gt;&lt;br&gt;'+x.desc,
            title: marketConf.dname,
            start: marketConf.start,
            map: map
        });

        // You should handle this by your class (the Marker)
        // or by configuration, like shown here 
        // so it's flexible
        if (clickHandler) {
            this.marker.addListener('click', clickHandler);
        }
    }
};


So your init function will be:

function init() {
    maps_1 = new Map('maps_1',{"lat" : 32.93,"lng" : 85.06},12);
    maps_2 = new Map('maps_2',{"lat" : 70.88,"lng" : 45.97},12);
    maps_3 = new Map('maps_3',{"lat" : 60.49,"lng" : 38.00},12);
    maps_4 = new Map('maps_4',{"lat" : 85.85,"lng" : 89.63},12);

    performPostAjaxRequest("ajax.php", function (data) {
        maps_1.addMarkers(data);
        maps_2.addMarkers(data);
        maps_3.addMarkers(data);
        maps_4.addMarkers(data);
    });
}


And loadJson will change to be more general purpose:

function performPostAjaxRequest(url, cb) {
    var xmlhttp = new XMLHttpRequest();
    xmlhttp.open("POST", url, true);
    xmlhttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
    xmlhttp.send(data);

    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4) {
            cb(JSON.parse(xmlhttp.response));
        }
    }
}


This is just a first step, a lot more could be done, the purpose of OO is to design the objects you need as classes, and put all the logic that is connected with those objects inside the class.
You should not have a function that know how to attach a Marker object to a Map object. Instead, the Map object should know how to do that. You just need to know that Maps need Markers.
When you define a class it's important to start the name with a capital letter, to distinguish a class constructor from a normal function.