In my experience having more than three in a lab group can offer additional challenges. Keeping the roles simple and specific helps students and groups stay focused.
Cut out these circles and use magnets or adhesive to post on your wall or board. I love to use these circles with magnets on my whiteboard. I have student names laminated with magnets as well and can quickly organize my groups and assign roles using these circles. If you don't have student names on magnets, you can always just write their names next to the assigned lab role. I have provided small and large circles to accommodate your needs.
Check out my store for the Lab Roles Poster to post on your wall for a great visual to help students stay on track. I also have have a listing for Group Circles that I use when organizing my lab groups.