Most of those clients will be swapped out because of sitting idle and reading pages or getting links and testing them then going back to get more links or read an article.

So out of those 4000 how many would be active?? Unless he has ajax scripts automatically updating. Snow storms and other intensive usage hooks/apps/mods.

So you have to be careful in how may chars and rows from mysql are returned and how it is sorted and displayed.

So depending on whether he has taken the time (which most don't) to go through the admin panel cpu savings sections and hooks/apps/mods to see what can be done to optimize usage he can definitely be forced to keep getting bigger servers and it never stops.

You can allow one ip to get 20-350 but why? Use iptables and limit each ip to 2 max. So many ways to stop paying so much and so few want to take the time to learn them.

So my suggestion to him is to check and see in his logs what the usage is coming from ..

Determine in mysql what can be done to correct it? New indices, return smaller amounts of data in sort change sort routines , stop ajax updates.

If server determine if you have IPs using 50-100 connections at one time and stop it.

Find if any one script or or app/hook/mod is overwhelming the system.