To balance the system load, you can use more than one IIS, pointing to the same GN4 database.
Important note about system time
In case you use more than one IIS, the time on all IIS computers must be synchronized. The best is to use the central time server to sync time of all IIS computers and also of the database server.
Fixes in version 1.6
Page preview failed in versions prior to 1.6 when using two IIS on the same database. The page preview creation could fail when using two (or more) iis on the same database, now fixed.