I have been hitting my body parts at a max of 6 reps, for muscle hypertrophy...
But what happens when you do even lower reps (2-4) at 90% of your max - does this promote growth even more or not?
it depends....what i think is the most important thing in hypertrophy tranning is the time of the set in seconds...so if you would do between 20 and 40 secs thats usually the range u wanna be in so if u can hold that weight for 20 seconds in movement for 2 reps try that .....cause 2 to 4 reps done in an explosive way will give u some benefits ..like nerveous system ....I try usually to mix it up ..
Going that heavy all the time could eventually cause some sort of muscle tear or injury. I'll max out once in a while but my working sets are always at least 6 reps minimum.
staying at 5 and under will give you strength gains w/o too much size increases. I know alot of sprinters who train this way to increase power while keeping weight down.