1.040 Original Gravity (OG) is a low original gravity beer, meaning it will be a small, sessionable, low abv beer. Nothing wrong with that, but that's the beer you brewed.
1.020 Final Gravity (FG) is typically too high for this type of beer and its probably not done fermenting yet. Let it go for a while longer and take another measurement, perhaps. Rouse it and raise the temp a little if you think your yeast just quit too early. If you can shave off a few more gravity points, that translates to a little higher ABV.
My advice is to not focus on alcohol content - brew a good beer.