She will do her homework tomorrow at 4 o'clock
She will be doing her homework tomorrow at 4 o'clock
Which one is correct?

Question

Answer

Both are correct depending on context.

Answer

They can both mean the same but also have a slight difference. 
They can both mean: 'She will start her homework at 4.' That said, the first one is clearer to demonstrate that meaning. 
However the second one can also mean that 'she' started her homework before 4, and is still doing it at 4, and will likely carry on for some time after.

Hope that helps.

Answer

Posted twice.