如何生成一个独特的4位字符串

I'm looking for a way to generate a (fairly) unique (non auto-incrementing) 4-digit string using the numbers 0 - 9 for each digit using C#. I can validate uniqueness and generate another number if a dup is found. I had thought about basing the number somehow on the DateTime object's Ticks property but am having a difficult time putting the pieces together.I'm looking for a way to generate a (fairly) un