Place 4 balls between each base, have 4 girls at the pitchers mound, on Go all girls run and pick up one ball and throw it to the next base. They then run to the center torch the pitchers mound and run to the next set of balls.  

Example for one girl: At the mound I run to the balls between home and 1st, pick one up and throw it to1st run back to the mound then on to the balls between 1st & 2nd, pick one up and throw it to 2nd, run back to the mound then on to the balls between 2nd & 3rd, pick up and throw it to 3rd, run back to the mound then on the balls between 3rd & home, pick one up and throw it to home and run to the mound to finish 1st.

The fielders at each base are now the runners and the runners have a break while a new crew fields balls at them bases.