Don't ask the question in the title. Also, provide more information.
Your question is equvalent to asking "Why does this equation use 5:
5x^2 + 2x + 3 = 7?".
I'm assuming that this is for a cashe implementation? The reason why it has 4 select inputs is because it wouldn't work otherwise. If you used 3, then you're obviously missing a place where a hit could be found. Though I probably wouldn't impement it as described.