String s = "123456789695478";
char[] c = s.toCharArray();
StringBuilder sb = new StringBuilder();
for (int i=0; i<c.length; i++) {
    if ((i+1)%4 == 3 && i+1<c.length) {
        sb.append(c[i+1]).append(c[i]).append(" ");
    } else if ((i+1)%4 != 0) {
        sb.append(c[i]);
    }
}
System.out.println(sb);