You have a 4000 psi machine. On the chart look along the top and you’ll see “psi”. The numbers below are how many gpm the nozzle puts out. So, since you have a 4000 psi machine, and your surface cleaner has a 2 nozzle bar, you want to follow down until you see “2” (2 nozzle x 2 gpm = 4 gpm which is what your machine puts out). Once you find the 2 follow it all the way to the left and you get your nozzle size. The gpm won’t always be exact. In your case instead of 2 it’s going to be 2.12. 2502 is the correct size. (25 is the degree or fan of the spray. 2 is the nozzle size. that’s where 2502 comes from). If you were to use the 2504 each #4 nozzle puts out 4 gpm. (Follow the number 4 to the right and up to the 4500 psi. It’s 4 gpm). If you used a 4 gpm in each nozzle that’s 8 gpm total which is twice as much as what your machine puts out.
I’d probably take it back and tell the guy he should’ve sold you a 16" As mentioned above they recommend 4" per each gpm. Either that or bring your machine too and tell him to get it to work. If it doesn’t work correctly he pretty much has to take it back as long as you haven’t had it all that long. If some place sold me something that didn’t work correctly I guarantee they’re going to make it right. Most decent businesses would be more than happy to make it right.