how to implement return url functionality for login page in asp.net mvc 4?

Simple solution could be as follows
Your controller get method will be like the following method taking the returnUrl as input parameter and setting it to the viewbag.

    [AllowAnonymous]
    public ActionResult Login(string returnUrl)
    {
        ViewBag.ReturnUrl = returnUrl;
        return View();
    }

Now in your MVC view you will be setting the ViewBag.ReturnUrl to ReturnUrl parameter of beginForm as shown below.

@using (Html.BeginForm(new { ReturnUrl = ViewBag.ReturnUrl }))

Now in your controller post method you will take the your LoginModel as well as returnUrl as an extra parameter.

    [HttpPost]
    [AllowAnonymous]
    [ValidateAntiForgeryToken]
    public ActionResult Login(LoginModel model, string returnUrl)
    {
        // your login login logic would be added here as well.
        if (ModelState.IsValid))
        {
            return RedirectToLocal(returnUrl);
        }
        // In case of failure form displayed again
        ModelState.AddModelError("", "The user name or password provided is incorrect.");
        return View(model);
    }

Following method is called in above controller method to make sure that the returnUrl is local url.

private ActionResult RedirectToLocal(string returnUrl)
        {
            if (Url.IsLocalUrl(returnUrl))
            {
                return Redirect(returnUrl);
            }
            else
            {
                return RedirectToAction("Index", "Home");
            }
        }