See: Rating System
The term Four-Plus, or (4+) came from this scale as many Farris channels and Donors exceeded the highest rating on the scale, which was 4.0, but there existed no technique for measuring how far they exceeded that upper limit.
The diagram and details of calculating the Proficiency Rating (PR) can't easily be copied into the wiki. There is a complex formula and details on the Simegen.com domain, found in the fanzine A COMPANION IN ZEOR # 16. http://www.simegen.com/sgfandom/rimonslibrary/cz/cz16/UNTO_OUT.HTM
(Source: Index card file. "Ratings of Channels and Donor" --- Jacqueline Lichtenberg interview with Linda Deneroff -- from the fanzine AMBROV ZEOR #3) Jacqueline has written articles which explain how much power goes into the rating of TN-1 as opposed to other ratings.
(Source: Index card file. Sime Surgeon http://www.simegen.com/sgfandom/rimonslibrary/surgeon/SURGEON1.html) Graded on a logarithmic scale of 4.0. Only a handful of channels, mostly Farrises, officially rated off the top of the scale. Testing standardized --- Digen would have to take 4 months off from the hospital, hence not for cripples and unusual types like Digen. Testing drove a channel as close as possible to death by Attrition in order to establish his psychological breaking point --- also meant to teach channel his personal limit, so he'd know to allow sufficient leeway in his work.
Proficiency Rating test for a Donor is to serve the Need of a channel in attrition, satisfactorily --- then they are given the PR number of the channel. Donors rated Four Plus-4+ need only spend time with 4+ channels.
People changed and grew, so in fact these numbers weren't particularly accurate if it had been years since the channel was last tested and rated. Channels grew fastest in the years after changeover.