Using a clustered environment will give you load balancing and failover. Your users will have an environment with higher uptime in general when you have more than 1 HW to utilize if 1 HW all of a sudden does not work ok. The different cluster parameters available in QV will give you the opportunity to use only 1 unique application in the whole environment, but the application can reside on different HW within the cluster day to day. You have to choose the "right" cluster parameter in the QMC for your needs.
It is possible to use 4 single QV servers without running the cluster part, but you will not gain for ex failover if you run into a HW crash.
If you run your environment without the cluster part, you will not be able to tell the customer that they are running on a business critical prepared QV environment.
If you run 4 QlikView Servers in a non-clustered setup, what do you intend to gain? The cost will be the same. You will have to run 4 license setups (meaning that users cannot migrate from one server to the other without moving their CALs first). And you may need four different AccessPoints... I think you better keep the clustered non-load balancing setup that you currently have.