How big server do you need if your traffic is 1K user at the same time

Hi,

I have multiple forums running on the same server with around 40K signed up users on these forums.

  • Daily user access averages around a 1.5k to 2.5k users
  • Daily views averages around 20k to 26k views
  • Daily user to user replies averages around 3k to 5k

I am expecting this traffic to go much higher in future.
I have DB and Redis running on a separate servers
All servers have 4GB of RAM assigned to them.

Could someone give me a rough estimate of what kind of resources will be adequate this kind of traffic ?

Thank you