1 mg is the equivalent of 1000 micrograms or mcg or ug. In the metric system a microgram is 1/1000 the weight of a milligram (mg), in other words it takes 1000mcg to equal 1 mg.
The last answer told you to: assuming the same pharmacy produced both doses, in other words, assuming that what you have is actually 'folic acid', then 400mcg is the same as .4 milligrams which is...