How do you recover if you lose track of what you're measuring?
Going forward, as you say, it's best to use either a separate bowl/ramekin/etc. for each ingredient, or to save on dishwashing, use one larger bowl and put each ingredient in its own spot in the bowl so you can see them and figure out what's missing. 

In this specific case, though, where you've forgotten if you're at 1/3 or 4/3 cup of sugar, you could weigh the existing mixture and try to figure out by weight how much is already in there:
1. Tare an empty pot.
2. Pour mixture into empty pot and weigh it; pour it back into original pot.
3. Weigh (using tared pot) an identical amount of water (then pour it out).
4. Weigh 1/3 cup of cornstarch (and pour it back into the box).
5. Weigh 4/3 cup of sugar (and put it back).
6. Add the weights from #3, #4, and #5, and subtract the weight from #2. 
7. Weigh out sugar to match the total in #6.

This is a silly process unless you've already added something expensive (or inconvenient to replace). I've gone through exercises like this when I've added saffron to some stock, or it's midnight and I don't want to go to the grocery store for a new box of baking soda.