There must be a reason of why there's no 4th unit. It wasn't featured in the previous maps of Mato and it's also not mentioned in this meeting, so the reason is that it doesn't exist.
Could it be because "four" can also be pronounced as "death"? (in Japanese that is, as both are "shi")
gah. this becomes more and more repetitive. New characters appear, suddenly monsters attack as a change to showing off how badass the new characters. Then new stronger enemies appear, the new op character suddenly so hopeless and finally MC saves the day.. Hurrah!