Need help with weight of Alexander III drachm

[Basileus Nikephoros Antialcidas]

Just got this Alexander iii drachm from a reputable seller. For me it looks ok but in the description of the coin the seller states that it weights 3,81g. It is also 18mm. For a coin that worn is it a normal weight, because I red that the normal drachm weights around 4,20g. Thank you everyone in advance!

[Altamura]

Your coin seems to be Price 1528 minted in Abydos, Troas.
Here you find a lot of examples of that type: http://numismatics.org/pella/id/price.1528?
As you can see, a lot of them are underweight, some also below 4,0 g.
So your coin is on the lower side of the statistical bandwidth, but still ok  .

Besides of that, it sounds as if you don't have the coin in hand yet. If so, then weigh it yourself when it arrives. Sometimes dealers make errors as well.

Regards

Altamura