Step 1: Understand the idiom "order of the day."
This idiom means something that is typical, prevalent, or commonly occurring in the present time or situation.
Step 2: Analyze the options in context:
(1) Something that is negligible: Incorrect — it suggests insignificance, which contradicts the idiom.
(2) Something that is a must: Close, but not quite accurate.
(3) Something that is stylish and attractive: Incorrect — unrelated.
(4) Something that is very common or important: Matches the meaning exactly.
Hence, the correct answer is (4) Something that is very common or important.