On the first day, the library used 4 students * 5 books = <<4*5=20>>20 books.
On the second day, the library used 5 students * 5 books = <<5*5=25>>25 books.
On the third day, the library used 6 students * 5 books = <<6*5=30>>30 books.
On the fourth day, the library had 120 books - 20 - 25 - 30 = <<120-20-25-30=45>>45 books remaining.
The library can help 45 books / 5 books = <<45/5=9>>9 students on the fourth day.
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