If you're using OC 703 or equivalent type traps, and you can't afford to go top to bottom, at least make them 6" deep. 4" is minimal for traps but 6" top to bottom would be good but at least 6. If you still can't afford to make enough to get at least one 6" in almost every corner, it's better to have more 4" panels straddling more corners than 2 6" panels on 2 corners.
I'd put them on the floor or right at the corner where the ceiling is. It cuts off the corner where two perpendicular walls and the ceiling meet.
I'm no pro. Hopefully someone with more knowledge can confirm if I'm wrong or right. I built 8 panels, varying thicknesses and sizes and placed them at corners and on 1st reflection points and broadband absorption areas in my studio/living room/dining room.