Generate 4 Character String

I'm creating SMS application for notify users about important activities. but the problem is there is no way to track for reply messages for relevant sms. I'm sending and receiving SMS from same number.
What i thought was generate 4 Character string and sending with the message like

<SMS message><space><GeneratedKey>

When user send reply with that generated Key value

<GeneratedKey><space><SMS message>

Using that key value I can find the reply messages for given SMS.

Since that key is need to be easy to enter I thought to generate sequence from both digits and characters and key length is 4.

each character sequence goes like.

0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ

generated keys should be like below.

0000<br />
0001<br />
0002<br />
...<br />
...<br />
0009<br />
000A<br />
000B<br />
....<br />
000Z<br />
0010<br />
....

Before sending SMS i'm going to add record to table with next key and after that I'm sending message with that key value.

Experts please give your thoughts on this approach and also can you please help me to generate next key value from c#?

Tags: C# (C# 4.0)