What’s better 5.5 gpm or reg 4.0 4000 gx390 Pressure washer?

5.5 GPM is better. A higher flow rate will allow you to clean faster because you will be using more water per minute.

Also, a higher flow rate means that you can use a lower pressure nozzle, which will be safer and less likely to damage the surface you are cleaning. For example, if you have a 3100 psi machine with a 4.0 GPM flow rate, you can use a 0 degrees nozzle and get the same cleaning power as someone with a 5.5 GPM machine using a 30 degrees nozzle.