Question 916353
There are 7 ways to choose the 1st letter. 
For each of the 7 ways to pick the first letter,
there are 6 ways left to choose the 2nd letter. 
For each of the 7*6 ways to pick the first 2 letters,
there are 5 ways left to choose the 3rd letter, 
For each of the 7*6*5 ways to pick the first 3 letters,
there are 4 ways left to choose the 4th letter.

In all, that's 7*6*5*4 = 7P4 = 840 ways.

Edwin</pre>