A uniform beam on a central pivot. Each weight makes a torque τ = r·F = m·g·r. The beam balances only when counterclockwise torque equals clockwise torque (Στ = 0).
Try: set both masses to 4 kg but slide the right one closer to the pivot — the beam tips left because torque depends on distance, not just weight. Then find the right distance that re-balances it.