If you fear you're going to fall below 4mmol having 20g of carbs as suggested will do the trick. 5 teaspoons of sugar = 20g of carbs. Just read the packaging of food products to ascertain carb content.
I once had a hypo of 2.6, I got back to 5mmol within 15 minutes, I ate a few buscuits
So testing around 20 minutes later should do the trick.