There is a dramatic difference in maturity between the two grades. My 4th graders are playing with dolls and other toys. If it is a school decision, then I would suggest that you maintain as much structure as possible. Make everything as organized as possible and let the children know what is expected day one. Teach as many subjects as possible thematically since the content for those two grade levels is sooo different.
Centers are a good idea since it lets the students work independently.
Have a teacher center so that as many students as possible see you daily. Support the students as much as possible and collaborate with 4th and 5th grade teachers. I taught out in LA, California. It was a common thing out there to have split classes. I seperated the class into two sections to start with.