To add, "at" is the preposition indicating point in time, and "about" is the adverb indicating how specific it is. You couldn't say with clarity, "I'll be there about 4 hours" as this would confuse the meanings of "in about 4 hours" and "for about 4 hours". Obviously "for" and "in" are the prepositions you need to choose between there. Similarly, "about" is not replacing "by" in "by 4 o'clock", for example, it's an addition to approximate the time and "by" and "at" are the prepositions. You need one or another to indicate whether you are saying "by about 4 o'clock" or "at about 4 o'clock", which have different meanings.
Especially in conversation, some people drop "at" as it seems "at" and "about" are replacing each other but actually they're not.