The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is:
“Clearance of at least three times the diameter of the largest reinforcing bar; three times the maximum size aggregate; or 2 in., whichever is least”
In your case with No. 5 bars, the 2 in. (50 mm) minimum probably controls and would then provide a 2.6 in. (64 mm) center-to-center spacing. No. 5 bars at 4 in. spacing can be shot properly with proper materials and technique. An ACI-certified shotcrete nozzleman will have had education that explains how this configuration or even closer spacing can be shot properly. Experienced shotcrete contractors doing structural concrete walls do this type of work routinely with excellent results. If you are concerned about encasement of the reinforcing steel in the back curtain of steel you may consider erecting only the back curtain of steel, shooting the wall out to the location of the outer curtain of reinforcement, erecting the outer curtain, and then shooting out to the final surface. Experienced shotcrete contractors have shot in thick sections (36 in. [900 mm] thick or more) with heavy reinforcement (No. 11 at 4 in.) using this technique.