documentclass{article}
usepackage{tikz}
usetikzlibrary{arrows} 
usetikzlibrary{positioning}    

begin{document}
    begin{tikzpicture}[
        > = latex',
        every node/.style={
            draw,
            minimum width=2cm,
            minimum height=0.8cm,
        },
    ]
%
node (S1) {Step 1};
%
node[below=of S1] (S2) {Step 2};
node[below=of S1, left=of S2] (S3) {Step 3};
node[below=of S1, right=of S2] (S4) {Step 4};

node[below=of S2] (S2a) {Step 2a};
node[below=of S3] (S3a) {Step 3a};
node[below=of S4] (S4a) {Step 4a};

node[below=of S2a] (S2b) {Step 2b};
node[below=of S3a] (S3b) {Step 3b};
node[below=of S4a] (S4b) {Step 4b};

node[below=of S2b] (S2c) {Step 2c};
node[below=of S3b] (S3c) {Step 3c};
node[below=of S4b] (S4c) {Step 4c};


node[below=of S2c] (S2d) {Step 2d};
node[below=of S3c] (S3d) {Step 3d};
node[below=of S4c] (S4d) {Step 4d};


draw[->] 
(S1) edge (S2)
(S2) edge (S3);

end{tikzpicture}
end{document}