I have four devices authorized, iTunes will not allow me to authorize a PC so I can download my purchases...keep getting error messages, You cannot authorize more than 5 computers...

Did you try "Deauthorize All"? See the following from Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

(Note that you will only need to authorize computers, not iPhones or iPads)

To deauthorize a computer that you don't have access to, you have to deauthorize all of your computers. Then authorize each computer that you have again. You can deauthorize all computers only once a year.

1. Open iTunes.
2. Choose Account > View My Account.
3. Sign in with your Apple ID and password. Forgot your Apple ID?
4. At the top of your Account Information page, go to the Apple ID Summary section.
5. Next to Computer Authorizations, click Deauthorize All.
6. Authorize each computer that you use again.