You got 4 POPs in Voaxoms IV because you have the 'Advanced Colonization' tech (colonies start with 2 POP) and because your colony governor Yani Nai's colonization desire for that planet gave you +2, resulting in 4POP. In the next turn, the POPs will get back to 3, which is the planet's population capacity.
Reasonable explanation... and completely counter-intuitive. It would make no sense to send more colonists to a planet than could be supported in the first place, when I saw this it bothered me as well. The population should start at three, which is maximum in these cases, not 4/3 only to be reduced a turn later. It's a small point, but It looks bad.
