Do a forum search on MK IV ... there was a post just a few hours ago about this and all the details elude me. Best I can say is that MK IV should be available from the Ferengi Merchants and possibly cargo ships you run into in Sector Space.
Keep in mind you must be of sufficient rank to equip MK IV equipment. Higher level equipment requires higher rank so verify you have the appropriate rank before spending the credits. This can be seen in the equipment destripction in the upper right hand corner.