What would be a 7-8. Would it be a small

Since small is the next size up from 4 Toddler, I would think that a small would be a 5-6. My guess is that a Medium would closer to a 7-8. Unfortunately, Levi's does not give a size chart. I even clicked on a few other Levi's tops and hoodies to see if there was a size guide and there isn't one for those items, either. Sorry that I can't give you a definitive answer and just a guess.

What would be size 16 from the size chart? (Chest 33”, waist 30”) - is it XL?

Generally, Girls XL is a (14-16), so I would guess it is since that is the largest size before going to Juniors; and this is listed as "Girls" jacket. That is my best guess. Hope that helps!

What's the difference between 4t and size 4?

Since it is a Girl's jacket, I would say No Difference. The only other size 4 is in Women's or Juniors and this is for "Girls". So, I would look at the biggest size on the scroll down tab and you will see how high it goes up to see that it is ALL Girls sizing. I am going to order another one for my granddaughter in a size 4 this time - the last one was a 2 - and she is a 4Toddler which is also 4T. Also, this is an amazing jacket - really cool and extremely well-made. After being worn al lot, it shows no real signs of wear. Awesome and versatile. Looks so cute over dresses and with skirts and leggings.