  We continue to study twistor spaces on the connected sum of four complex
projective planes, whose anticanonical map is of degree two over the image. In
particular, we determine the defining equation of the branch divisor of the
anticanonical map in an explicit form. Together with previous two articles
(arXiv:1009.3153 and arXiv:0705.0060), this completes explicit description of
all such twistor spaces.
