Encrypting plain text out of MD5 hashed password

Hello App security experts, I need little help/direction in developing password security module. I am using MD5 hash algo to store hashed password (with salt) in DB and when user login, again creating hash (with same salt) and matching with the one in DB and allowing user to login if both hash matches. It's working fine till this point. Now the requirement is to not allow continuous block of 4 chars of previous password in new password. In this case, I need to get the plan text of previous password to match with new password to check if user has supplied continuous 4 chars in the new password or not. Now the issue is how to get the plan text of hashed password (stored in DB). I checked some of the articles and can't really make out if plain text can be retrieved out of hashed password. Any help or pointers or any alternate approach will be appreciated.