First 5 letters of the alphabet are {A, B, C, D, E}

The 4 letters can be chosen as possible:

5 * 5 * 5 * 5

The number are not repeatable, so the 4 numbers can be chosen as:

10 * 9 * 8 * 7 since we have one less choice with each pick

Grouping letters and numbers together, we have the following serial number combinations:

5 * 5 * 5 * 5 * 10 * 9 * 8 * 7 =

**3,150,000**