Length $1$ strings are :- $“a”$
Length $2$ strings are:-$”aa”,”ab”,”ac”$
Length $3$ strings are :-$ “aaa”,”abc”,”abb”,”acc”,”aac”,”aab”$
Length $4$ strings are :- $“aaaa”,”aabc”,”abca”,”abcb”,”abcc”,”aabb”,”aacc”,”abbb”,”accc”,”aaab”,”aaac”,”abbc”$
So total no of string of less than or equal to 4 is =$1+3+6+12=22$.