Writing tip: At 4 AM, be consistent

 Which is correct?

• 4 AM
• 4 AM (AM is supposed to be in small caps, but durned if I can achieve this)
• 4AM
• 4 am
• 4am
• 4 a.m.
• 4 A.M.
• 4 A.M. (A.M. in small caps)
• 04:00

They are all correct. It all depends which style guide you follow. Just be consistent!

I'm inclined to avoid a version involving small capital letters: they're not easy to convey online. I made two attempts to show small caps with HTML and CSS, above, without success. No joy. The universe has delivered a message: Small caps are just too fiddly for everyday business English.

Plenty more tips like this one in the Contented Modern Punctuation course!