It would be better to wait.. This suggests a conditional recommendation, implying that waiting is a good idea based on the expected outcome (when the weather improves) "Would" is commonly used for polite suggestions or conditional situations. Example: "It would be safer to drive in the morning when the roads are clear." "May" expresses possibility but does not strongly suggest that waiting is better. Example: "It may be better to call first before visiting." (Here, the possibility of being better is uncertain.) "Could" implies a possibility, but it is weaker than "would." Example: "It could be better to take the bus instead of driving." (There are multiple possibilities.)

The first three are very good choices. "Should" is also grammatically correct. In fact, any modal verb would be correct. Since all are grammatical, the choice comes down to what meaning you want to convey.

All four options are valid grammar.