Webservice collects a multidimensional json

I'm making the following ajax call to my webservice.

var datos = {"user_id":"2","body_param":"body","title_param":"title","arr":{"1":1,"2":2,"3":3,"4":4}};
console.log(datos);
$.ajax({
    type: "GET",
    crossDomain: true,
    dataType: "json",
    url: "http://localhost:24234/api/SendPushNotification",
    success: {
        function(result) {
            Console.log(result);
        }
    },
    data: datos,
    headers: { 
       'Token': 'ccTtUKQS32PjzMYQEzH2OUQN8nJpsCUXg7iWRYA7cEpAJLoNEWCKr2NcQyFUbdV4' 
    }
});

As you can see, multidimensional data is sent. The problem I have to receive. I've tried arrays, lists, ... and nothing.

Public Sub SendPushNotification(user_id As Integer, body_param As String, title_param As String, arr() As String)
   Try

The only one that arrives empty is arr, the rest receives its values properly.