It is possible for that to happen. The system tries to balance the number of whites and blacks overall. It looks like prior the 4 blacks you had 5 whites in a row also.
The color choice algorithm also has to contend with balancing out what color your opponent gets also so runs of a color can be more possible because of thatt too.
How is it supposed to work? Is it normal that I have played the last 4 games as black?