I know that Android 3 and 4 use MTP on Windows to allow access to the storage, I have tried the developer switch and the only connection you get is still MTP.. I really need a drive letter to enable several programs that I want to use with my ASUS slider (this question has a good chance of being needed on many Android 4 systems). is there any app or driver that will allow mass storage / drive letter access to Android 4
Unfortunately, it's a big problem. Google apparently decided to go MTP only route since it does not require unmounting the storage from the Android side when you connect it to your computer. So:
You cannot assign a drive letter to an MTP device. Here's my answer on superuser.com explaining why.
There is a program named Alt Mounter that supposely solves this problem, and it requires a special software both on the phone and on your computer (Windows/Mac). It is beta and I never tried it.
If your phone is rooted, you can install a Samba Server on your phone, and simply map a network location to a drive on your computer. No USB is needed, simply access your phone using a drive letter via Wi-Fi. This is the method I use, and it works fairly well for me.