How would you reach try to Topik 4 if you have only 1.5 years?

Hi!
You are at TOPIK level 2 and have about 1.5 years to reach at least TOPIK level 4, how would you do it? And what resources would you use?

Is it possible to get there by using only free resources? If yes, which ones?

I'm looking for a study plan with which I can reach my goal, so I'm very thankful for any serious answers :)