wait a minute...
Most websites have a disclaimer about mistakes in the wording. Northern's reads "Northern is working hard to ensure that the information provided on the Northern site or in any Northern catalog or advertisement is accurate and up-to-date but due to the risk that the information may be compromised by software or procedural errors, Northern does not guarantee the accuracy or completeness of the information provided on this site or in any Northern catalog or advertisement ."
This is their escape clause for errors like this. If the manual states 3000 PSI, I would bet that is all it is rated for. In that case, applying 4000 PSI will cause the equipment to fail and likely void the warranty. At the very least, you will be into a long fight with them to get any warranty coverage.