Print the maximum number of composite numbers that sum up to n.

Example:
given n = 81

81 can be represented as 18 * 4 + 9
so n can be represented as 18 4's and one 9. So together 18+1 = 19 number of composite numbers

Output: 19

The least composite number is 4.
so when n is represented as multiple of 4, we get maximum number of composite numbers.
When n is not divisible by 4 we get a remainder 1,2,3
When remainder is 1 then
excess = 1
to make number divisible by 4
we cannot subtract 4+1 = 5 as 5 from n as 5  is not composite number
so we take two 4's = 8 and 8+1 = 9 is next least composite number and hence we subtract 9 from n to make n divisible by 4.

When remainder is 2 then
excess = 2
to make number divisible by 4 we subtract 4+2 = 6 from n.

When remainder is 3 then
excess = 3
we cannot use 4+3= 7 as 7 is not composite number
so  we take two 4's = 8 and 8+3 = 11 is not composite number
so  we take three 4's= 12 and 12+3 = 15 is next least composite number and hence we subtract  15 from n to make n divisible by 4.
and 15 can be expressed as 9 and 5

#include<stdio.h>

int main()
{
   int n,ans= -1,r;
   scanf("%d",&n);
   if(n >= 4)
   {
       r=n%4;
       if(r == 0)
            ans = n/4;
       else if (r == 1)
       {
           if(n >= 9)
                ans = (n - 9)/4+1;
       }
       else if(r == 2)
            ans = (n - 6)/4 + 1;
       else if(r == 3)
       {
           if(n >= 15)
                ans = (n - 15)/4+2;
       }
   }
   if(ans == -1)
         printf("Not Possible");
  else
         printf("%d",ans);
}

Efficient Solution
If the number is 5, 7, 11 then it is impossible to represent it as sum of composite numbers.

#include<stdio.h>

int main()
{
   int n,ans= -1,r;
   scanf("%d",&n);
   if( n == 5 || n == 7 || n == 11)
          ans = -1;
   else if(n >= 4)
   {
       r=n%4;
       if(r == 0 || r == 2)
            ans = n/4;
       else if (r == 1 || r == 3)
                ans = n /4-1;
    }
    if(ans == -1)
         printf("Not Possible");
    else
         printf("%d",ans);
}