Assuming that there are 600 G traditional Catholic SSPX faithful, four bishops are not enough. I remember Bishop Williamson would fly from Rochester, WI to Australia for confirmations. That is insane. He dreaded that flight; I think it was 22 hrs.
The SSPX should issue a directive, "We will have absolutely no communications with Rome until Rome returns to the Faith and the valid sacraments."
Considering the amount of distance that these bishops will be travelling, I say they need at least 2 bishops in North America, at least 2 in Europe, 1 in South America, 1 in the Philippines, 1 in Australia, at least 1 in Russia (if Russia has an SSPX Mass). We are already at 7-10 bishops.