Answer
$12,338,352$
Work Step by Step
The number of arrangements of $n$ objects in $r$ slots where each slot can be any one of these objects is $n^{r}$.
Number of company names represented by 4 letter
$26^{4}=456,976$
Number of company names represented by 5 letter
$26^{5}=11,881,376$
Total: $456,976 + 11,881,376 = 12,338,352$
