ClockHubClockHubzero four hundred
Military time writes every hour of the day as a two-digit number from 00 to 23, then appends the minutes as two more digits. 0400 therefore means 04 hours and 00 minutes past midnight — 4:00 AM on a 12-hour clock.
Read aloud it becomes "zero four hundred". A zero minutes value is spoken as "hundred", which is why whole hours end in "hundred"; minutes below ten take an "oh" in front, as in "seventeen oh five" for 1705.
Twelve-hour clocks split the day at noon. Hours 00 through 11 are AM, hours 12 through 23 are PM, and the two boundary hours are the ones people get wrong: 0000 is 12:00 AM and 1200 is 12:00 PM. 0400 sits on the AM side of that line, which is why it converts to 4:00 AM.
Going the other way is the same arithmetic in reverse. Take 4 and AM, drop the suffix, and add 12 to any PM hour — then watch the two twelve-o’clock cases, where 12 AM becomes 00 and 12 PM stays 12. 4:00 AM becomes 0400.