String  as = "0005123450003123000812345678";

while (as.length()>0){
    int strCount=Integer.valueOf(as.substring(0, 4));
    String res=as.substring(4, 4+strCount);
    //out print your string
    System.out.println(res);
    as=as.substring(4+strCount, as.length());
}

------解决方案--------------------
Java code

public static void main(String[] args){
        String str = "0005123450003123000812345678";
        while(str.length() > 0){
            int len = Integer.parseInt(str.substring(0,4));
            String sub = str.substring(4,4+len);
            System.out.println(sub);
            str = str.substring(4+len);
        }
    }

------解决方案--------------------

String strs="0005123450003123000812345678";
while(strs.length() > 0){
    int str1=Integer.parseInt(strs.substring(0,4));
    System.out.println(strs.substring(0,str1+4));
    strs=strs.substring(str1+4);
}

红色部分 看您是否需要前面的'000'