The numbers indicates the fineness or purity of the gold. .999 Fine (three nines fine) means that it is 99.9% pure, while is .9999 (also known as “four nines”) Fine is 99.99% pure.
According to Goldmart.com, they're both designated as 24 Kt. gold., but four nines is highly refined which means it has very little impurities like silver, iron and copper are in the total mix as compared to three nines fine.
The difference between solid-gold and gold-plated jewelry is related to metal composition: While solid-gold jewelry is made of a gold alloy that is consistent throughout the piece, gold-plated pieces are made of a non-gold metal that is just covered with a thin film of gold.
Check out some tips on how to find out whether a piece of jewelry is solid gold or gold plated at Jewelrynotes.com.