Scaling back just the base malts (fermentables) will lean a style to have more mouth feel body and make residual unfermentable sugars more noticeable without the alcohol to "cut" them down. These changes can also be countered by water dilution.
If you had a 4% ABV recipe and scaled back the fermentables 25% it should still be pretty close to style and enjoyable.
But if you want to geek out to try and simulate the original beer closer you can dilute with water. Let's use a 10 gallon 4% ABV recipe scaled to 3% ABV as the example. It loses 25% of its ABV but 1% ABV is only 0.5% of the fluid volume. Now adding 6.4oz of water to 10 gallons wouldn't change much of anything because alcohol as far as mouth feel and flavor "cuts" more than water. From experience adding water equal to the the volume the missing alcohol would occupy gets it pretty close. 1 cup water + 1 cup alcohol = 1.5 cups fluid. So for this example add 19.2 fl oz or 2.4 cups of water, post boil or even post fermentation. Doesn't sound like much and will reduce the recipes ABV by 1.5% going from 3.0 to 2.955, But it does work and this is just for the geek factor, you will probably be very happy with just reducing the base malts.
Irish and sweet stouts have a BJCP style baseline of 4-4.2 ABV. But all styles can be a Sessions also.
As a side note, consider brewing second runnings (partygyle) of a buddies all grain stout brew. I've found that a 7-8% ABV brew can have second runnings that can make great sessions with very little tweaking. Look up how scottish/60 is historically made for more insights.
