When selecting A4 paper, the most critical factor is Grammage (GSM); 80 GSM is the international standard for high-quality office printing, while 70 GSM is often used for high-volume drafting to save costs. You must also verify the Whiteness/Brightness level (CIE), where a higher CIE (e.g., 160-170) indicates a premium blue-white shade suitable for external documents. Additionally, ensure the paper has high Opacity (at least 90%) to prevent 'show-through' during double-sided (duplex) printing, and check for moisture content (4-5%) to prevent paper curls and jams in high-speed copiers.