I'm creating a 4x6meter outdoor billboard that includes an image and vector logo (rasterized) in photoshop. I was working using a smaller size for design purposes, but now the client has approved a final design and wants to go to print. I need to know what DPI/PPI I should export it as.

Additionally, I was given advice by a friend to make it 30 dpi and work in centimeters (so 400x600cm) then let the print house blow it up to 4x6meters, is this true?