"mil" actually means "a thousand" in latin. More crucially, a millimetre is a thousandth of a metre. This means that 4 metres is in fact 4000 millimetres...
Since 1 meter equals to 1,000 millimeters because 'milli' means 1,000.If there are 4 meters, then that means multiply it by 4 to the top and bottom.Therfore you would get 4,000 millimeters in 4 meters