Final answer:
Fibrinogen is the clot forming factor that constitutes 4% of plasma proteins. It is one of the three major groups of plasma proteins and is essential for blood clotting.
Step-by-step explanation:
The clot forming factor that constitutes 4% of plasma proteins is fibrinogen. Fibrinogen is one of the three major groups of plasma proteins, along with albumin and globulins. It is produced by the liver and plays a crucial role in blood clotting.