They indeed did have 0.4 at the start of 2018.I'm sure I've seen more than 4 on past visits- more like 6 or 7. So possibly/probably there were a couple more deaths of females between 2014 - 2018.
PZ lost their 2 old breeding females (b. 2001 and b. 2005 resp.) a few weeks apart in March 2018, so only their 2 female offspring born in 2009 were left.
Now, it may just be they lost one of the 2 remaining bongo after that ... as Gigit mentioned. This should be Adina (the one that passed on in April 2019).