The students have been placed into 4 ability-based groups. I use a mix of things to ability group them. Our district does NWEA(MAP) testing, I use my observations, and my leveling assessments (to see those check out my Teacher's Pay Teachers page).
Group 4 is always my lowest group. They get to start with me right away. This group often needs to do a lot of review. Many days, I stretch out their teacher group time to be able to get in the review.
Group 1 is my highest group. They are often above grade level. While I still teach them the 4th grade common core, I assess them earlier than the other groups and we usually move on to above grade level concepts by the end of the unit.
Groups 2 and 3 are my at and around grade level groups. They may need a day or two of review, but then they are ready to move on. Depending on the unit or common core standards one or both of these groups may need to review to get to 4th grade standards.
After 10 to 20 minutes (depending on the schedule for the day, the students interest levels, and needs) we switch what they are working on. Hence the name for this section, rotations. I tell the students to, "rotate", or "rotation ____".