I teach 5th grade and see 3 different groups of students during the day. I divide my scoreboard into 3 chunks and, like Annette suggested, have them compete against each other as well as themselves. For example, the winning group gets to play mind soccer but each group is playing against themselves for extra recess everyday!
Have you read through the Model Classroom e-book yet? If not, check out the part on Prove It! I used it with my math students last year and saw great gains.
The "Big 7" will apply for all four of your classes. I'd suggest you do what's comfortable for you and tweak things as needed.
Good luck and let us know how it goes!
Tiffany
WBT Intern, 2011-2012