Can you play a wild card on an existing Canasta if it becomes the 4th wild card in the meld?
It depends on which rulebook you follow.
Most commonly, Canasta rules say that a legal meld cannot contain more than 3 wild cards:
A meld is valid if it contains at least two natural cards of the same rank - aces down to fours inclusive - and not more than three wild cards.
Melds must be valid at all times, whether during the initial meld, or when adding to an existing meld:
A player may add additional cards to a meld by his side, provided that the melds remain valid (having no more than three wild cards).
However, some rules teach a variation on this rule, and say that rather than being limited to 3 wild cards, a meld cannot have more wilds than naturals:
It is possible to add a wildcard to a column of cards of any rank on the table. The wildcard is then seen as a card of that rank (but the value remains unchanged). However, there may never be more wildcards than natural cards within one meld.
Under this rule, a meld with 4 naturals and 4 wilds would be a valid meld.
Under most circumstances, these 2 rules function the same; they prevent a Canasta from being made with 3 naturals and 4 wilds; and prevent melds such as 1 natural and 2 wilds.
Note that if you need to meld an extra wild in order to go out, you can always just add it to any meld you already have; it seems like it would be a very rare situation that a Canasta is the only place you have to add it to.