This is the five races flag but there are only four stripes? Why?
Because there are no Tibetans in Manchuria?
A bunch of Mongol governments in the 30s and later Mengjiang used four coloured flags without the black.
Thought OTL Manchukuo did use all five colours, thought changing the meaning of red-blue-black colours (from Han, Hui (as in Muslims rather than the modern understanding of the term) and Tibetan to Japanese, Han and Korean) (yellow and white stayed for Manchus and Mongols).