Heian Yondan is not more difficult that Heian Sandan. The kata utilizes more kicking with back fists thrown simultaneously, more reinforced blocking using both hands, and large use of the elbows and knees, but unfamiliar techniques are not more difficult, they are simply unfamiliar. Practice will bring these techniques under control just as it has for all of the techniques the student has had to master up to this point. The most difficult movements are generally considered by novices and many older students to be the back stance and the side snap kick. These challenging techniques require flexible hips and ankles.
Strong knees are also helpful.
Being 8 years old is also helpful. These techniques were not structured so that anyone could learn and perform them. Most adult novices find these techniques to be impossible to ever perform to the level demonstrated in books and videos. It bothers me that the back stance and the side snap kick are so difficult. Because of that, they are given an inordinate amount of training attention by students and instructors that is better utilized elsewhere.