I have a 3.4.7 test site that I made some posts on and tried to upgrade it to 4.0.8.  The upgrade process went smooth, however, here's what the site looks like after the upgrade:

I tried to login into the site and received the 500 error.  Any ideas?  I uploaded the 4.0.8 files twice and it didn't seem to solve anything.  Don't know where to find and error codes in 4.X

That's your test site looking like that right? Did you check file paths prior to the upgrade? I've had the same issue in the past with my test site and it was because I forgot to change the file paths to reflect the test environment prior to the upgrade.

The paths would remain the same since I'm only upgrading the test site.

I had this. I fixed it by checking the config_global.php settings, checking the "ACP > Files > Storage > Config > Edit" values, and running the support tool to rebuild the themes. Tricky to do when the CSS is missing, but it is possible.

I can't log into the site due to this 500 error.

Can you not login straight to the ACP ? Try /admin on the URL.

Can you not login straight to the ACP ? Try /admin on the URL.

I did try that, but when you click on a link, it just refreshes the page.

Does it just refresh to that page or refresh to the front end ?

Temporarily remove any .htaccess from within the /admin directory (if present) to see if that improves it.