The stability of a synovial joint is maintained by a combination of structural and dynamic factors. Among the options provided:

- Option 1: Ligamentous support plays a crucial role by physically connecting bones and preventing excessive or abnormal joint movements. Ligaments provide passive stability to the joint.

- Option 2: Muscle tone surrounding the joint contributes significantly to stability through dynamic support. Muscles contract to hold the joint in place and control movements, enhancing joint integrity during activity.

- Option 4: Bony congruence of the articular surfaces refers to how well the shapes of the articulating bones fit together. This structural alignment provides inherent stability, particularly in joints like the hip and ankle.

On the other hand,
- Option 3: Synovial fluid viscosity primarily functions to lubricate the joint surfaces, reducing friction and allowing smooth movement. While important for joint health and function, the viscosity of synovial fluid does not directly enhance joint stability.

Thus, the factor that does NOT primarily enhance the stability of a synovial joint is synovial fluid viscosity.

Reference: Gray's Anatomy, 42nd Edition, Volume 1, Chapter 6: The Joints—Structure and Function (Page 650)