The total crew onboard is listed as 4 and the cfg file says 4, you can add four members and they indeed fly.
The issue is they do not display in the lower right corner of the active vessel. Which is weird.
It may be that you used the internal stock module for ...
INTERNAL
{
name = Mobile_Processing_Lab_Int
}
Which only hold two crew members.
Seems cosmetic but it would be nice to have the two other members displayed. Without the portraits they cannot eva.