What new to learn to push from 3 star to 4 star

from the start of div 4, it was easy to tackle first four questions as those were tricky i jumped to div 3 there i saw only 80-90 people submit div 3, problem 4 as it was way above the level and the speed with which we solved first three questions decided our rank. now that i have reaced div2 i need to do atleast three questions, any one who faced with problem before can share their experience or tell me how to surpass this and able to solve the 3rd problem of div 2.