kdel[i_,j_] := If[i==j,1,0]

Cijkl[i_,j_,k_,l_] := lambda kdel[i,j] kdel[k,l] + 
shear( kdel[i,k] kdel[j,l] + kdel[i,l] kdel[j,k])

Cijkl[1,1,2,3]

index= ConstantArray[1,{4}]

indexMap[p_] := Module[{index= ConstantArray[1,{4}]} ,
Which[
p ≤ 3,{p,p},
p == 4,{2,3},
p == 5,{3,1},
p == 6,{1,2}
]
]

Cpq[p_, q_] := Module[{index= ConstantArray[1,{4}]} ,
{index[[1]],index[[2]]}= indexMap[p];
{index[[3]],index[[4]]}= indexMap[q];
Cijkl@@index
]

Cpq[2,4]

MatrixForm[Table[Cpq[p,q],{p,6},{q,6}]/.{lambda-> ym sig/((1+sig)(1-2 sig)),shear -> ym/(2(1+sig))}]