Deficient and scarce mean there is not enough of something or that something is lacking. Scant and meager mean that there is not very many of something, barely sufficient. That does not necessarily mean that the amount is deficient, though most of the time it does. If I say the desert has scant forests, that does not necessarily mean too few - just that there are few of them. The four are related and the meanings overlap at times, but they are not quite the same.