gvpack (graphviz) not preserving labels of the individual digraphs

Question

There are four dot files (cluster_0, cluster_1, cluster_2, cluster_3) which need to be merged into a single dot file.

The code of the individual dot files is as follows.

digraph cluster_0{
sortv=1;
penwidth=0;pencolor=transparent;
label="1A";
rankdir=LR;
 node [fontname = "times-roman-bold",fontsize=12];
        edge [fontname = "times-roman-bold",fontsize=12,arrowsize=0.75];
    1 [label="1:A"];
    2 [label="2:C"];
    3 [label="3:G"];
    3 -> 1
    1 -> 2
    3 -> 2
    2 -> 3
}
digraph cluster_1 {
sortv=2;
penwidth=0;pencolor=transparent;
 label="1B";
 node [fontname = "times-roman-bold",fontsize=12];
        edge [fontname = "times-roman-bold",fontsize=12,arrowsize=0.75];
    rankdir=LR;
    11 [label="1:A"];
    12 [label="2:C"];
    13 [label="3:G"];
    14 [label="4:T"];
    14 -> 11
    11 -> 12
    12 -> 13
    12 -> 14 
    13 -> 14
}
digraph cluster_2 {
    rankdir=LR;
  sortv=3;
penwidth=0;pencolor=transparent;
  label="1C";
 node [fontname = "times-roman-bold",fontsize=12];
        edge [fontname = "times-roman-bold",fontsize=12,arrowsize=0.75];
    21 [label="1:A"];
    22 [label="2:C"];
    23 [label="3:G"];
    24 [label="4:T"];
    25 [label="5:A"];
    24 -> 21
    21 -> 22
    21 -> 25
    25 -> 22
    22 -> 23
    23 -> 24
}
digraph cluster_3{
        sortv=4;
penwidth=0;pencolor=transparent;
        label="1D"
 node [fontname = "times-roman-bold",fontsize=12];
        edge [fontname = "times-roman-bold",fontsize=12,arrowsize=0.75];
    rankdir=LR;
    31 [label="1:A"];
    32 [label="2:C"];
    33 [label="3:G"];
    31:c -> 31:w
    32 -> 31 [dir=both,len=1.25];
    33 -> 31 [dir=both,];
    32 -> 32
    33 -> 32 [dir=both,];
    33 -> 33
}

To generate a merged single dot file ...I ran the following command.

neato -Gstart=5 cluster_0.dot cluster_1.dot cluster_2.dot cluster_3.dot | gvpack -array_u4 | neato -n2 -s -Teps -o merge.eps 

merge.eps came perfectly well but the labels of the individual graphs went missing. How can we preserve the labels of the individual graphs in the output merge file ? Can anyone help to fix this ?

Here I am attaching the single individual file where the label is shown I compiled the individual file with the following command.

neato -Gstart=5 cluster_0.dot -Teps -o cluster_0.eps

Also I am attaching the merge output file which removed the labels of the graphs.