The only thing that I would add is that sometimes the post in the die that causes the central opening in the number 4 breaks off causing the 4 to be filled in during the striking process. With the amount of wear on your '84 dime, it is hard to say whether metal was smooshed there during an old hit or left the mint with it already in place like that.
"If you climb a good tree, you get a push."
-----Ghanaian proverb
"The danger we all now face is distinguishing between what is authentic and what is performed."
-----King Adz