For clearing the fourth trial the rewards Elena got were the best among all the other rewards. That instantly proved the fact that this trial was most important among all the trials for the creator of the trial ground, immortal Serene Mountain.
First she got an "Origin Transformation Pill". This was without a doubt a pill that could wage wars. This pill had the ability to transform a person's initial talent and potential to make it something greater. If a person took this pill he was sure to at least reach the initial saint realm. A pill with that kind of effect was invaluable to anyone below the saint rank.