According to Ext4 preview documentation, we should get the user object as parameter to success callback of Model.load method:

Code:
//get a reference to the User model class
var User = Ext.ModelMgr.getModel('User');

//Uses the configured RestProxy to make a GET request to /users/123
User.load(123, {
    success: function(user) {
        console.log(user.getId()); //logs 123
    }
});
However, if you run the above example you get an undefined user variable.