Would / Will

[jack]

What do these mean?

1. That would be great. (Is this more polite? Also, is this a conditional sentence? If so, this is incorrect right?)
2. That will be great.

3. If you can do that, that will be so nice.
4. If you can do that, that would be so nice. (Is 'would' more polite?)

4. If you could do that, that would be so nice. (Are 'could' and 'would' more polite?)
4. If you could do that, that will be so nice.

[Mister Micawber]

Yes, 'would/could' are more polite than 'will/can'. The former distance the speaker from the possibility of the favour by indicating that it is conditional upon the listener's acquiescence.