The University of Chicago's School Mathematics Project has a comprehensive math program for kindergarten through sixth grade that includes answers to Common Core math problems for fourth grade. While there are other great resources available, such as AdaptedMind.com and CommonCoreStandards.org, UCSMP teaching methods are more informed.
The project helps students bring their performance in mathematics up to standards, according to the UCSMP website. UCSMP accomplishes this by compiling data from around the globe on various teaching methods in mathematics. This data is standardized and re-evaluated regularly in school districts where the UCSMP is present, to the benefit of students and educators alike.