Within IPS 4 you can use an external database as a login handler. You would do this from the following location in your admin CP
System>Settings>Login & Registration
From here you can then set up the relevant details needed for connection
In the first section you will need to enter all the details to connect to your mysql data. Ensure these details are 100% connect and can be connected to from the server where your IPS installation is located.
This is where you will set up the information related to where your users are located. You will need to provide the Table and at minimum either an email address or username column. You can of course use both if you are using both methods of login.
You will need to add in the password column and tell the system how this password is stored. If this is encrypted you will need to ensure you provide the correct Encryption type.
There is also an additional clause which will allow you to add to the end of the query which returns users. So for example, you may have users who you do not want to have access and have a column named 'active'. This will allow you to add active=1 and it will add to the end of the sql statement 'WHERE active=1"
You will need to set up which authentication type is being used, and whether or not to allow Admin CP login with this method.
Finally, there are various options available to determine what happens in various situations, such as changes are made on the local account to a display name or email address.