1. The problem asks for the number of different character sequences of length three to four that can be formed from a four-letter alphabet \{A, C, G, T\}. 2. Each position in the sequence can be one of 4 letters. 3. The number of sequences of length 3 is $4^3 = 64$. 4. The number of sequences of length 4 is $4^4 = 256$. 5. Total number of sequences of length 3 or 4 is $64 + 256 = 320$. 6. Therefore, the correct answer is 320.