Final answer:
The 4th muscle that completes the quadriceps femoris group, alongside the vastus lateralis, vastus medialis, and vastus intermedius, is the rectus femoris.
Step-by-step explanation:
The quadriceps femoris is a group of muscles located on the anterior (front) side of the thigh and plays a crucial role in extending and stabilizing the knee. This group consists of four distinct muscles: vastus lateralis, vastus medialis, vastus intermedius, and the rectus femoris. While there are 3 vastus muscles mentioned, which are located in different parts of the thigh (the lateralis on the lateral aspect, the medialis on the medial aspect, and the intermedius deep to the rectus femoris between the other two), the 4th muscle that completes the quadriceps group is the rectus femoris. This muscle is situated on the anterior aspect of the thigh. Together, these four muscles converge into the quadriceps tendon that inserts into the patella, and they are crucial for movements such as walking, running, and jumping.