As we know, graduate schools usually require three letters of recommendation. Some require exactly three and some don't specify this(and even in such cases, it may be still better to submit only three).
Since missing one letter can be really disastrous to applications. I wonder if it is appropriate to filling the information of four professors to request four letters, just to reduce the risk of missing letters.
For example, in the application process, I will fill in four professors' names and email addresses but due to some unexpected situation, only three professors submitted their letters before the deadline. How would graduate school consider this situation?(I think they should have been aware that I have requested four letters)
Will they be happy with the three letters they have already received and never wait for the fourth one, or unhappy because I requested four but one of four professors hasn't submitted the letter?