What is the difference between 弯身, 弯腰, 俯身, and 屈身?
弯 (curve; bend) vs. 直 (straight)
弯身 = curve/ bend your body
Although bend your body from your waist (弯腰) is the most common way to bend your body, there are other postures that can be called 弯身.  For example, to get under and pass a police tape, you can curve your waist to one side (face sideway)
~
俯 (face downward)  vs. 仰 (face upward)
俯身 = body faces downward
Although bending your body from your waist (弯腰) does make your upper body face downward, there are other forms of 俯身. For example, you don't need to bend your waist when you 俯身撲向水池 (launch your body faces downward into the pool)
~
屈 (bend) vs.  伸 (stretch)
We use 屈 mostly for body parts that can be folded completely, e.g. 屈指一算; 屈膝下跪.
屈身 (bend your body) is similar to but not as commonly used as 弯身
~
弯腰 (bend your waist)
Since human's waist naturally bends forward. 弯腰 refers to this specific movement/ posture -- not bend sideways or backward