Periodical properties of multidimensional polynomial generator over Galois ring. IV

$m$-dimensional polynomial substitutions of Galois rings consisting of $q^n$ elements and having the characteristic $p^n$ are investigated in this paper. The maximum cycle length in such substitutions is $L_m(R)=q^m(q^m-1)p^{n-2}$. Substitutions that contain an $L_m(R)$ length cycle are called full-length cycle substitutions (FLC-substitutions). A method permitting to construct FLC-substitutions is proposed. The number of substitutions that can be constructed by this method is estimated. The obtained results are applied to the synthesis of polynomial shift registers with a given cyclic structure.