The first condition means that you have to choose from the $9000$ numbers between $1000$ and $9999$. The second one, assuming that your alphabet contains $25$ characters, means that you the letter can be any of $23$ different ones. With no further restriction, the number of combinations is
$$9000 \cdot 23.$$
If you prefer, the first part you could also view as 4 separate digits, so that there are 9 combinations for the first one, and 10 for the remaining three, which nets you
$$9 \cdot 10 \cdot 10 \cdot 10 \cdot 23$$
Edit: Okay, after seeing the other answers and counting thrice, I now see how to arrive at $26$ letters instead of $25$ ...