First, according to Interpump (the owners of General Pump USA) the TS2021 pump is rated at 5.6 gpm at 3000 psi. The latter rating is the resistance of the head, packings and seals to the pressure and the former is the flow rating at a given pump RPM (1450).
Second, as a marketing move General Pump USA (when in private hands) warranted the same pump for 3500 psi claiming to have made modifications to the packings and seals (also the TS1521 and T9211). Please note that the repair kit numbers did not change (Kit 1 and 6), so the the claim I feel is somewhat dubious.
Third, Interpump purchased General Pump USA and did not change the ratings of these pumps to correspond to the rest of the world's production.
Fourth, to directly answer your question, in order to get more flow from the pump you can overdrive it (run faster) by putting smaller pulleys on and upsizing the nozzle appropriately. However, when you upsize a nozzle you will lower pressure.
Fifth, according to the formula used by Cat Pump, 18 hp will give 5 gpm at 3504 psi (#4 nozzle will give 4.7 gpm), within the rating of the pump). At 4000 psi the motor is only capable of providing 4.4 gpm (#4.5 nozzle). Unless the manufacturer of the pressure washer is providing their own warranty I would imagine that General Pump will not warrant this pump at 4000 psi. I would suggest calling them for details.
Sixth, anything over 3500 psi is required by safety legislation to have steel fittings at
ALL points of the machine from pump oulet, to unloader, to hoses, fittings and even coil construction. Schedule 80 pipe (most commonly used) is technically rated at 700 psi but our industry has used it to 3000 psi for years and recently to 3500. A typical engineering rule of thumb is static burst strength being at least 4 times working pressure. Most every Schedule 80 coil is tested to 14,000 psi static burst, therefore 3500 psi X 4 is 14,000 or within the safety margins; 4000 psi working is only a 3.5:1 safety margin. Close but are you willing to risk it? The coil should be at least Schedule 120 or better still Schedule 160 to provide the safety margin you deserve.
Seventh, the formula used to determine performance is Flow X GPM / 1460 = Electric motor HP. Electric motor HP X 1.5 = Gas engine HP. This is a realistic performance calculation based on experience (again from the Cat Pump information). Some manufacturers use the Engineering Theoretical performance calculation of Flow X GPM / 1100 = Gasoline engine HP. This, I feel, is flawed because it uses maximum theoretical performance numbers for all components and therefore does not represent, again my opinion, real world actual.
Eigth, FLOW will always be of more value than pressure. Pressure has it's value to dislodge stubborn dirt but flow will apply chemical faster and rinse faster the more you have. Personally I would run the pump at 1450 RPM to get 5.6 gpm (see the Wiz's chart to determine the correct pulley size) and lower the pressure to 3000 PSI (#5.5 nozzle = 5.7 gpm).
Hope this helps?
Cheers, Michael