Here is my guide. I use an Epson inkjet. As mentioned above, dpi is a bit irrelevant. Wjen I want an A4 printout, 30 cm along the long edge, I use 1600 as the resolution along the long edge. This is fine for landscapes, and will show no pixels on a good photo-quality print. If you have lots of straight lines in the image, particularly diagonals, push this up to 2000. A render of 4000 along the long edge for a domestic printer is, I think, excessive. I have just had an A2 printout made on a plotter, brilliant quality. This is 60 cm along the long side, and a render at 4500 gave no visible pixels on the print.