i see that you say "everything works in 4.0 except the login".

couple of questions

1) have your registered .net 4 framework on the machine that this is running on?

         command prompt: C:\Windows\Microsoft.NET\Framework\v4.0.30319  -->  aspnet_regiis.exe

2) have you configured IIS properly on the machine that this is running on?

3) is "login/usedcarlogin.aspx" (logon folder) actually in the root?

what gets me is this:

Requested URL: /UsedCar/Account/Login

and your web.config

<forms loginUrl="login/usedcarlogin.aspx"

perhaps use

<forms loginUrl="~/login/usedcarlogin.aspx"

and

defaultUrl ="~/used_cars.aspx"