10 times smaller, for a 4m x 3m billboard a 40cm x 30cm render @ 100dpi will translate to 4x3 at 10dpi which is what billboards are printed at... since this is in your clients office on a wall, just ask the print shop how high in the dpi department they can go, and maybe render split images, because youll be having problems working on extremely large images. for example 24.000px wide render would translate to 3.5GB rgba tiff image and photoshop only supports 4gb tiffs or 2gb psd files, so that's a no go.
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