I will try to help but first I need to ensure I understand what you want,
So below is my version of what you want.
In a particular Category there are 5 Downloads. Four of the Downloads may be downloaded by the Public and will show a Download button. One of the Downloads will show its title but will not show a download button and thus will not be downloadable by Public. For a Registered (logged On) user then all 5 items will show a download button and be downloadable.
Is the above interpretation correct?
If it is then it is quite possible. The scheme uses the standadard Joomla permissions and the Joomla View Access Levels. Briefly the Access level allows a user to see something whilst Permission allow actions such as Create, Edit, Download.The containing Category and all the Downloads need Public view Access Level which is the probable situation now.
Esure that the component level Permissions for Download for the Public User Group (UG) are set to Inherit. The default configuration of jD is that the Component Permissions for Download are set to Allowed for the Public UG. Do not use Deny.
The other permissions need to be set on the Downloads themselves
The Category and the four publically downloadable Downloads need Download Permission for the Guest
UG with the Public UG left at inherit. The fifth Download needs download permission set for the Registered UG.
I have not yet tried this scheme but will check out if you confirm my understanding of the 'challenge' is correct.