Here is a scenario :-
Sharepoint by default allows us to upload a file with 50 MB as the upper limit.But when trying to upload a file over 30 MB, the upload instantly fails with a 404 (Page Not Found) error.
One of the possible option is the server may be installed with URL scan version 2.5 or above.This problem is most
likely caused by a default setting in URLScan which limits the size of HTTP requests to 30 MB. The upload fails instantly because IIS cuts the connection as soon as it checks ContentLength header of the HTTP Request.
Navigate to the configuration file for URLscan.
This is usually is usually located in the directory C:\Windows[or WINNT]\system32\inetsrv\urlscan.
Open the file "urlscan.ini"
Change the setting of MaxAllowedContentLength to the maximum number of bytes you wish to allow. By default, URL restricts upload size to 30 MB:
MaxAllowedContentLength = 30000000
Restart IIS by typing "iisreset" at the command prompt