Problem solving skill is a basic skill that students should acquire both in mathematics education and their daily lives. This research aims to examine fourth-grade elementary school student skills in solving four-operation problems prepared in different structures according to the situation of the unknown. In this research, on the one hand, students' success in solving four-operation problems prepared according to the unknown (initial, change, and final unknown) was examined, and on the other hand, problem solutions were analyzed according to Polyo's problem solving steps. This research is a survey model from quantitative research methods. The sample of this research consists of 150 students studying in the fourth grade of two public primary schools in Amasya city center in the 2021-2022 academic year. The "Problem-Solving Achievement Test" developed by Bayar (2022) was used as a measurement tool in this research. Based on the findings obtained because of the analysis of the data; it was determined that students were significantly more successful in problems with unknown outcomes than in problems with unknown both initial and change. Besides, although the students were successful in the step of understanding the problem in the problem-solving steps, they were less successful in the steps of planning, executing the plan, and controlling the plan, respectively, compared to the step of understanding the problem. A positive relationship was found between all the variables in this research, albeit at a moderate and low level. It would be beneficial to revise the problems in the textbooks and classroom practices within the framework of the results.