The Common Core emphasizes and explicitly mentions utilizing Algebra in the lower grades. Enclosed are 4 worksheets with 10, 15, 20 and 25 problems per skill respectively. Since there are 4 skills (2 for grade 4, 1 for grade 5 and 1 for grade 6) that amounts to worksheets of 40, 60, 80 and 100 problems apiece.
Take a look at the PREVIEW and see if this is something you can use. I, personally have used these problems in my several courses designed for at-risk kids.
[The problems in All WORKSHEETS and PREVIEWS are printed in difficulty (1 [easy], 2 [not so easy], 3 [proficiency] up to 4 [mastery]) order within a skill. The actual difficulty level appears slightly to the left of the DK box of each problem in a very small font. Also, instructional references appear on worksheets but are optional on quizzes.]