In Present tenses^x, Difference between^x, Conditionals^x

Abbie wrote: 2. Why would you want to do that? Here, you are using a conditional, 'would', because although the plan has been made, there is still the opportunity to change it - it is not definite. "I might dye my hair green?" (I'm thinking about it, but I haven't decided) "Why would you want...

Would you please tell us now the difference between 2) and 3) , then 3) and 4) ? (2) If that kind of tsunami should come to Japan tomorrow, Tokyo will be devastated. (3) If that kind of tsunami should come to Japan tomorrow, Tokyo would be devastated. (4) If that kind of tsunami were to come...