$ sudo apt-get update
$ sudo apt-get install libmcrypt4 

libmcrypt4 is installed in your system.

Make ensure the libmcrypt4 package were installed using the commands given below,

$ sudo dpkg-query -l | grep libmcrypt4 *

You will get with libmcrypt4 package name, version, architecture and description in a table

libmcrypt4 package basic information: