3,000 or 4,000 PSI? You probably need GPM, not PSI.
Most people shopping for more pressure are short on flow and do not know it yet.
Someone calls wanting a higher-pressure machine. We ask what they are cleaning and how long it is taking. About two-thirds of the time the answer describes a flow problem.
The diagnostic question
Does the surface come clean where you point the wand? If yes, and the job is simply taking forever, you are short on flow. Pressure is breaking the bond fine — you do not have enough water to carry the result away and move on.
If the surface is not releasing at all no matter how long you sit on it, that is a pressure problem, a chemical problem, or a heat problem. Usually one of the last two.
Why the mistake is so common
PSI is the number on the front of the machine and in the advertising. GPM is buried in the specification table. People buy the number they were shown.
What this means for a purchase
A 3,500 PSI machine at 8 GPM will out-work a 4,000 PSI machine at 4 GPM on nearly every large job, and it is not close. Compare the pair on this site and think about the surface area of your typical job before you assume more pressure is the upgrade.
Before you buy flow, check what your water supply can actually deliver. An 8 GPM machine on a supply that gives you 6 is a machine that will cavitate.