Formula for this fork() related problem is
Total number of process created ={ (2n) -1)+1 }
where { (2n) -1} = number of Child Process and 1 is for Parent Process.
Now here fork() is called 4 times . That means n = 4 . so put n=4 in above formula we get total number of process created is 16.
Among which 15 =2n - 1 are child process. And 1 is parent process.
so answer is 16 here .