Create the entire four panel spread as one graphic 73.5"w x 35.875"h. (No bleed is necessary)

Panels:
Side Panels:
Will be cut to 18.375"w x 35.875"h
Center Panel:
37" wide, will be cut down the middle.
(2 - 18.375"w x 35.875"h panels)

34.5" x 9.625"
Add 1/2" all around for bleed.