Asking for more than three letters of recommendation from four professors to reduce the risk of missing letters? 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?
The two different application management systems that I've used at both a public and private R1 (intensive research oriented doctoral granting universities) don't care once you've hit the minimum required -- the application is marked complete if it meets the minimum number of items in that category even if there are other letters that might be pending. 

So it's a good strategy to request more letters than are minimally required because of the strong possibility of professorial flakitude.