The Explode the Code workbooks are fabulous. For your rising 4th grader, you might try Wordly Wise. Also, Horizons Math worksheets are a good supplement for reinforcement, as building a good math foundation necessarily requires repetition. IXL.com, though not a workbook, might meet your needs, as it gives feedback on answers and keeps a record of work completed. It provides good review for both math and grammar. Check the Homeschoolbuyers Co-op website for reduced pricing.