It depends on what you are doing. However, I'm pretty sure that potassium metabisulphite is the same as sodium metabisulphate, and you want potassium sorbate. But don't quote me on that because I always get confused. 4 days is not a long time for it to ferment, and 1.020 will give a pretty sweet cider. If that was your aim, then stopping it at that point is fine but the alcohol content will be lower than intended.
2 cups of sugar is a fair bit to add to the cider. A lot of ciders are just left to ferment with their natural sugars which doesn't give a huge ABV but in my opinion gives the best flavour.
Ciders are supposed to be left for several months to age to get back the apple flavour. It is quite important for a decent tasting cider.
The next time you do it, I suggest using a mix of apples (approx half bitter, half sweet and a few crab), juice them and throw in 1 campden tablet per gallon. Leave for 24 hours and then pitch your yeast. Leave it to ferment with an airlock until the bubbles have slowed right down and then rack to secondary (this is where airlocks become vital). Leave the cider for a while so it can start to clear. It will continue to ferment slightly, but getting it away from excessive amounts of lees will stop any off flavours. Then leave and rack every 60 days or every time you have 1/4 inch of lees or more. Do this until it is clear and then add campden. If you wish to back sweeten, add stabiliser.
Bottle and leave for around 3-6 months.
This is by no means the greatest cider recipe ever but it's a good base. And took me ages to write on this iPod.