Methane is about half as light as air which weighs a bit more than 1kg/m3
4000m3 methane must weigh around 2000kg, not 2kg.
Be also careful with the decimal separator: "." in English (or at least in the UK and US).
Pure chemists seem happy with grams, but if you plan to make also mechanical and acoustical engineering, I recommend to opt for Si units once for good, with kg and Pa. Whatever your choice, always check for g versus kg (and versus N).