I've been giving some thought to this one....I believe Melody is correct.....our previous answers were incorrect with respect to allowing repeats.....I think that the answer is WAY larger.......
I see this as a "slot" problem with these considerations........
1. Choose any 4 of of 7 "slots" and let each slot be occupied by any one of 26 letters. Notice that order isn't a consideration, since, for example, "slots" 1-2-4-7 are the same as 2-1-4-7 or 1-4-2-7.
2. For each arrangement above, the other 3 "slots" can be occupied by any three digit combination utilizing numbers 0 through 5.......there are 556 of these possible combinations.......(000 through 555).
So we have:
C(7,4)* (4^26) * 556 = 87,640,048,748,629,852,160 combinations !!!
Whew!!!....that's a pretty big number!!! (Note that the 4^26 part contributes to most of the "bigness".....)
Anyone else have any ideas on this??