Which one should I use in following sentence:
I need to set my alarm clock at/for 4a.m.
You probably want "for". I need to set my alarm clock at 4am: At four o'clock in the morning you will be setting your alarm clock. I need to set my alarm clock for 4am: At four o'clock in the morning the alarm will go off (and wake you, we hope).
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You probably want "for".
I need to set my alarm clock at 4am: At four o'clock in the morning you will be setting your alarm clock.
I need to set my alarm clock for 4am: At four o'clock in the morning the alarm will go off (and wake you, we hope).