I've been asked by my league members if I can increase the number of races to 2 per week - One on Sunday, One on Wednesday - at the same time as the current Sunday race is scheduled to start.
I'm targeting adding the ability to change your schedule for the Q1 patch but it was a late addition to the list. The option I will add will only be able to change it between seasons, not during a season.
With your situation I would recommend closing your current league and starting a new one based around the new specifications, since there are only four people in the league.