no different than not adding enough sugar, its meant to be drunk at 5.5% all strong beers suffer a but if you reduce the abv, split the batch if you have 2 small fv's, make up to 12 litres without the sugar split into 2 - 6 litre batches add half the sugar into 1 keeping the recipe standard at 5.5% then make the 2nd up to 8.25 litres which will make it 4%, you might have to get another yeast or make your sachet into a starter then split between the two, how to do this is on the website, not as scary as it sounds