#### Answer

475255

#### Work Step by Step

Let us consider a string of length n.
No. of choices: n
No. of options in each choice: 26
Therefore, no. of strings of length n = $26\times26\times ... \times26$ [n times]
=$26^n$
So, number of strings of length four or less = $26^1+26^2+26^3+26^4$
=($1+26^1+26^2+26^3+26^4$)-1
=$\frac{26^5-1}{26-1}$-1
=475255-1
=475254
Including the empty string, we have 475255.