Traditionally, you're supposed to have 60 cards in your main deck, and 15 cards in your sideboard. But let's say that I had 64 cards in my main deck and 15 cards in my sideboard. For game 2 & 3, could I reduce my main deck to 60 cards and effectively have 4 more cards in my sideboard, or do I have to keep my main deck at 64 cards?
You can in fact change your deck size after sideboarding, with restrictions. The Magic Tournament Rules, section 6.1 says
Constructed decks must contain a minimum of sixty cards. There is no maximum deck size. If a player chooses to use a sideboard, it may not contain more than fifteen cards.
And section 6.2 says
Players may exchange any number of cards between their deck and sideboard, provided that the resulting deck and sideboard are legal. There are no restrictions on the number of cards a player may exchange this way. Cards do not need to be exchanged on a one-for-one basis.
So, you can move a different number of cards into your sideboard and into your deck, but when you're done, you still have to have at least 60 cards in your deck and at most 15 cards in your sideboard.
In your specific case, you cannot decrease your deck size because your sideboard is already at the maximum size.