Quote Originally Posted by DanielP View Post
*None* of your usage screams anything crazy, but it is good to have redundancy. If you have a service impacting issue, having things split between several servers does mean that not all of your client base would be impacted.

Price wise your current budget fits you in a wide range of services. I'd recommend leaving yourself room for expansion and getting two slightly larger dedicated servers, and combine the 4 into 2.
I would have to agree with Daniel on this.
Another way to cut some costs depending on if you are tied to the panel or not is using something like Direct Admin. However with your amount of accounts two dedicated servers with either DA or Cpanel would be a great option. The nice part about this as well is there are less systems to manage cutting that from 4 into 2.

Centos as others said is ending life soon and it's best to make a good ahead of time. This is a prefect time to upgrade.
Regarding your budget good job it's nice to see a solid budget that should cover you quite well for a pair of pretty decent systems and support.