Preproduction environment should be exact replica of production environment. Though you don’t need to have DR setup for preprod and corresponding WAF and LB setup. It should have same infrastructure; same hardware in terms of machine, CPU, memory and peripherals.  This environment can be used as staging area before codes are moved to production environment. Also this environment can be used for performance testing. Because this   is replica of production environment and by doing performance testing in preprod, you can ensure that system will react similarly in production with the same load. So you can determine how interfaces will work in different load condition in production.

It is not mandatory to have preprod environment.  Many of server setup in reputed organization does not have prepod. But it is good to have specially when performance is paramount important and one should do performance testing before production migration of the interfaces.

You can simulate normal load situation in production using preprod .You can also simulate Disaster situation load by switching servers on and off.

In two nodes preprod cluster you can have 2 nodes in primary site. You don’t need DR site servers.

In four node clusters topology , you should have 4 nodes ;two node in primary and 2 nodes in DR site.