We are in the era of “searchable” documents. An attorney looking for the time of day is not likely to put in “ten” or “four” in his search. The English rules of the 1990s for times don’t work anymore. It is correct that English says to put the number into words with the word “o’clock.” We have moved beyond that rule.
…He arrived at ten o’clock. (…the way it was done in the last century…)
…He said they would be here at 4:00.
…I know it was getting close to 3:00 A.M.