Hi Ibrahim,
This must be some wrong or bad configuration settings of server. Because, your server seems to be of a good capacity so merely 4 users shouldn't add up this issue. Try to revisit the installation configuration and open file settings.
Also, does this service or meeting running since a long time ?
Memory is not a concern here as it is showing under control.
With similar server we have tested up to 80 users and max 30 videos in a room and then our kurento reaches to this %age.
Best,
Naresh K.