Heather,
I'll just move this post over to the Main Board, so more moms will see it. All new posts begin on the Main Board.
As far as your question goes, in your situation we'd recommend doing Bigger 4 days a week and then setting it aside on your co-op day. Then, on your next school day, you would pick up where you left off and go forward. This would mean that at the end of the school year you would have approximately 8-9 weeks remaining to complete of Bigger at the beginning of the next school year.
We wouldn't recommend trying to cram two days into one day or adding more to the days earlier in the week to get 5 days into 4 days. You'll lose the whole flow of the guide if you try this.
Once you get to the guides from Preparing Hearts on up, they are 4 day plans.
Blessings,
Carrie