Im always pushing heavy with 5 to 10 reps maximum and always to failure every set. I rest 2 days week , take my nap . I never use high reps training with afraid losing muscle and lose density.

Should we change workout every 4 weeks? 8 weeks or just when you stop improving?

Id say change every 8-12 weeks to hit the muscle differently from different angles etc and as a form of injury prevention like over use injuries in elbows or joints/connective tissues in general

Im doing things a bit differently now but i like to hit all the rep ranges all the time
Exercise 1 = 4-6 reps power movement
Ex 2 = 8-10 power accessory
Ex 3 = 10-12 stretch movement
Ex 4 = 12-20 pump/push blood movement