You cannot see the 2 unused devices, but you can do this:
At the place where you're viewing your iTunes account, and you see that you have 4 devices, you are given a choice to "deauthorize all". Accept that choice, then go ahead and authorize the MacBook and iPhone.
Note you are allowed to have up to 5 devices authorized at any time.
You won't lose any music on your computer. Authorizing and deauthorizing does not move the music, it only changes who can legally listen to it.
You have to authorize on the account on your computer -- you could not do it on the i-Devices even if you wanted to. If you have trouble hearing the music on your i-Devices, you may have to sync them.