The 4-foot measurement will not allow for the 6 Inch gap for the ramps. The PVC must be placed closer. This also causes the Drop Zone to be much closer to the back PVC perimeter, which can cause the balls to drop through the single hole to bounce out of the field easily. I suggest after the correct measurement is known to push the drop zone forward into the inner zone to help the balls from dropping out.
The GDC has found that if the middle of the field (where the mini arch gate is) is set to 37 inches, as shown in Appendix A of the Competition Manual, the 48-inch measurement of the outer zone can vary by +/- 3 inches, depending on how accurately the PVC is cut.
While the exact measurement of the outer zone is not critical, it is essential that the blue ramps are flush with the PVC to ensure that balls traveling from the inner zone to the outer zone must go over the ramp and not around it.
As long as the fields are consistent at your event, small variations in the outer zone depth will not impact gameplay.