Help and practice for 4th-grade level math are available through IXL and Go Math! Academy. Both sites offer help and practice for a variety of specific topics, such as multiplication, money, addition and measurement.
IXL is a free resource with unlimited practice work for each subject. There is a paid subscription for families that has more features, such as mobile apps, immediate feedback and audio lessons for math subjects.
Go Math! Academy is a product of Houghton Mifflin Harcourt and is a paid service, though there is a two-week free trial for the site. The site has practice problems, educational games and tutorial videos.