The "Rules for using Oksana" in her rule booklet state: "If you choose Oksana as one of your Heroes, her two Voivodes must replace one of your Troopers. Remember that a player's team must consist of three Heroes and two Troopers (unless otherwise noted)."
If you play Oksana in a Matriarchy team, you still have to use two Troopers. I think you have no other option than using four Voivodes.