In my opinion, the answer is a resounding YES! I would even walk the extra mile and say that 6 Bishops is the bare minimum. Assuming that there are enough qualified candidates to fill in the remaining 3 vacancies for Bishop, we should then consider the issue of SURVIVAL. If we are to believe and accept that a great catastrophe (Chastisement, WW3..etc) is going to happen soon (within the next 3-5 years), 100 years of Fatima comes into mind, then GEOGRAPHY becomes very critical. I would agree with Matthew that the number of priest becomes irrelevant when SURVIVAL is the issue. Common sense dictates that the Bishops would no longer to be able to travel freely if WW3 or any major catastrophe's would break out.
Hence, if we are able to have 6 Bishops then the distribution in my opinion would go like this: 1 North America, 1 South America, 2 already in Europe, 1 Asia and 1 Australia. For the purpose of survival, this would be a good set-up because let's say the Bishop in North America dies or gets killed, then the Bishop in South America would act as back-up and would have to consecrate a new Bishop for North America. Same procedure for Europe and Asia.
However, IF the 3 Bishops are hardwired in their decision that 4 is sufficient then here is my suggestion. In this scenario, we should consider 4 major areas which are Asia, Europe, North America and South America. Here are some possible approaches:
Plan A: If someone in North America gets chosen as the 4th Bishop, then either Bishop Williamson or Bishop Faure should be willing to go to and permanently stay in Asia.
Plan B: If someone in Asia gets chosen as the 4th Bishop, then either Bishop Williamson or Bishop Faure should be willing to go to and permanently stay in North America.
Plan C: Mix and Match, choose whatever combination you fancy :)
Bottom line: 1 Bishop in Asia, 1 Bishop in Europe, 1 Bishop in North America and 1 Bishop in South America.
PS. If you're gonna criticize me, please include your name and address, so I know where to find ya. ;-)