Just Quad Sig, no, but 4 Sigs with techs seems viable.
Time to remind everyone why I am not just neurotic for being a stickler about "meaning".
The "infamous" Quad Sigilyph deck that Tham Kennard used to win the Southeast Asia Regional Championship had six Pokémon; quadruple
Sigilyph and double
Mewtwo EX. There seems to be two definitions for "quad", and while it might seem contradictory to some of my other stands, I believe both are valid.
Definition A: A deck build around four copies of the same Pokémon, with no other Pokémon. See also "True Quad" deck.
Definition B: A deck build around four copies of the same Pokémon, where the Pokémon in question is of extreme significance.
Definition B needs work, because I wouldn't call a Deluge deck "Quad Squirtle" because while important,
Squirtle is being run to Evolve. On the other hand, "Quad Deoxys EX" might be a valid name for the strategy built around using Power Connect to boost an additional Pokémon being used as the deck's main attacker. Considering how many real world terms have divisions like this, I believe it preserves the usefulness of the word and is ultimately less misleading than forcing once definition or the other.
I believe the "true" Quad Sigilyph deck was never a legitimately viable strategy, and the version Mr. Kennard used won due to his skill and luck plus his opponent's decision to ignore the possibility someone would run such a thing. He read the metagame accurately, others didn't, plus if you've read his report, he honestly was fortunate as the opponent's prepared to bypass Safeguard. Note that I do not wish to diminish his win. That is the downside of how this game works; he may still have won if, for example his first round opponent ran two non-Pokémon-EX to attack with, and one was Prized.
As soon as we get
Silver Mirror I think
Sigilyph (Safeguard version) and
Latias EX could make a pretty solid wall deck. Most decks that can deal with the effect of one or the other can't also deal with
Silver Mirror... other than with
Tool Scrapper (or a similar card), of course.