After adding a presentation server 4.5 to your existing server farm (in this case 4.0). You get all sorts of performance problems. When running the QFARM /LOAD command you see that some servers are having a continues load of 10000 (Full Load). When checking those servers, you might find that no users are connected to that particular server. Strangely the counters where not resetted by rebooting that particular server. I solved it by reapplying the load evaluators.
According to Citrix this is a known issue in Citrix Presentation Server 4.0, this problem would not occur in a native 4.5 Farm. Here you will find a documents which reports known issue's that are solved in Presentation Server 4.5.