You could go with an extension of the current formula:

=IF(COUNT(A2:E2)=5,AVERAGE(E2-D2,D2-C2,C2-B2,B2-A2),"")
or use an array formula:

=IF(COUNT(A2:E2)=5,AVERAGE(B2:E2-A2:D2),"")
confirmed as an array formula with Ctrl-Shift-Enter.