If you scan it at a higher resolution than you need, you can enlarge your art without it getting blurry. A lot of home scanners can do north of 1200ppi, at least in one direction(cross scanning is often higher than step scanning) and may be able to software interpolate higher on scanning, into the 14,000ppi range, at least, according to my Epson documentation (don’t believe it.)
Going from A4 to A3, you want to double your print output resolution as you are scanning something at half the size it is going to print. IOW, if you want to print your A3 final image at 300ppi, when scanning your A4 art, scan it at 600ppi.
But as B noted, any small imperfections in your art are going to be doubled in size. When I scan my inked art, I always scan actual size, then reduce it for use. It also kind of depends on what you plan to do with it after you scan it. If the inking is a prelude to coloring digitally, you can muck around with the contrast curves to get rid of the paper tooth. Up to a point. There is a very fine line between where you can get a bright-ish white, and yet keep the integrity of your finer lines. If it’s solid work without a lot of feathering or crosshatching, you’ll be fine. Beware of erased pencil. You might have to go back in there and touch that up on scanning too.