What did 4-year-old Bruce Banner see in the room?

Bruce saw his father kill his mother.

When baby Bruce is born, David notices that, when he grows upset, his son's legs begin to ripple and bulge, turning slightly green. Realizing what he has passed on to his son, David works for years to find a cure, but his work is shut down before he can do so. Here, we encounter the first of the film's themes, inner turmoil. The regret and fear over what his son might become because of him drives David, instead of seeking help, to destruction. He rigs the lab to blow up and, returning home to kill the now four-year old Bruce, kills his wife when she intervenes. This scene is initially seen through Bruce's eyes in the form of a nightmare with him hearing his parents arguing fiercely in the kitchen with the door closed. Later on, when the scene is fully shown, David screams at the young Bruce in a fit of rage and anguish, underlying another theme: that of the dysfunctional father-son relationship. This theme is fully bared in a pivotal scene later on in the film.

Honestly, he probably doesn't really see or understand it fully at the time.