1 x 4GB refers to the number of 'sticks', it means you have 1 stick of 4GB RAM
if it was 2 x 2GB, then you would have 2 sticks of 2GB = 4GB ram (this is generally noted as a matched pair).
your laptop may come with RAM slots free, so you can manually upgrade the amount you have (normally laptops have 2 RAM slots, 4 if they are expensive ones). So you 'can' buy another 4gb soDIMM to get a total of 8GB RAM (that would then be 2 x 4GB).
BUT many would suggest you buy a 'matched pair' of 2 x 4GB (8GB total) as RAM sticks are like people, sometimes they dont get along ... buying in a matched pair garantees a happy marriage. you dont want them fighting eachother right?