there is a conversion factor of 18.To get from mmol/ l to mg/ dl multiply by 18.the other way round divide by 18. 4.0 mmol) is 72 mg/ dl.

In the US they use their own system - it is still "metric system" but the US measures the number of "milligrams" of sugar (physical weight) that would be diluted in a "deciliter of blood" (about 3.4 oz)

Pretty much the rest of the world uses a different measure - "millimoles" is a liquid measure (volume) so the "other system" compares how many "millimoles of sugar" are in a "Litre of blood" (33oz)

One is not better/worse, more/less accurate - just the Americans being different - like the rest of the world uses the Metric system and the US still uses the "non-imperial" Standard system they created themselves