At the time when production for season four was at its peak, production was halted as one of the crew members had probably been infected with the virus. Later, it appeared this wasn't the case. It does, however, have a consequence. The fourth season will receive three episodes less, only nineteen out of the originally planned 22 episodes have been produced.
The last two episodes are set to air on the 29th of April and the 6th of May. Whether or not the remaining three episodes will move to the fifth season isn't clear as of yet. This will result in an earlier goodbye for Skeet Ulrich (FP Jones) and Marisol Nichols (Hermione Lodge), who would both leave the series after this season.
