Gemara

109b - Question: Why did Chachamim enact to drink four cups? This is dangerous!

Beraisa: One should not eat two (or any even number of) things, drink two cups, clean himself twice (after eliminating)...

Answer #1 (Rav Nachman): Pesach night is "Leil Shimurim" - it is Shamur (guarded) from Mazikim in all generations.

Answer #2 (Rava): The cup of Birkas ha'Mazon joins for the good, not for harm.

Answer #3 (Ravina): Chachamim enacted four cups like free men drink - each is a Mitzvah by itself (they do not join).

110b: The general rule is - if one is particular about Zugos, the Ru'ach Ra'ah strives to harm him; if one is not particular about it, it is not so adamant to harm him. Nevertheless, one should be concerned.

Rav Dimi: A tradition from Moshe from Sinai teaches that one should be concerned for two eggs, two nuts, two gourds and two of another matter. Chachamim were unsure what the other matter is, therefore they decreed about everything.

Berachos 51b - Rav Asi: We do not bless Birkas ha'Mazon on a cup of punishments;

Rav Nachman bar Yitzchak, supported by a Beraisa: This refers to a second Kos (it makes one vulnerable to Zugos).

Rishonim

The Rif and Rosh here bring the question from the four cups and the answers given.

Rif (Berachos 38a) We do not bless Birkas ha'Mazon on a cup of punishments, i.e. a second Kos.

Rosh (Berachos 7:35) If one drank two cups he should not bless Birkas ha'Mazon on a third. He cannot concentrate because he fears being damaged.

Question (Ma'adanei Yom Tov 3): The Rosh did not bring any laws of Zugos in Pesachim because he agrees with Tosfos (see (5) below) that nowadays we are not concerned for Zugos!

Note: Perhaps the Rosh refers to one who fears Zugos, and even though there is no danger today, he should not lead Birkas ha'Mazon because he cannot concentrate. However, we could ask the same question against the Rif, and we cannot give this answer according to the first Pirush of R. Yonah (38a DH Mai) who explicitly says that he is prone to be damaged.

Rashbam (DH Kol): Surely, if one is not particular about Zugos he still should be concerned somewhat. If indifference to Zugos gave total protection, Chachamim would not have taught about Zugos at all!

Tosfos (Chulin 107b DH Hasam): Nowadays we are not careful about Zugos, because that Ru'ach Ra'ah is not frequent among us.

Me'iri (109b DH b'Kamah): In the time of the Gemara many people were drawn after boorish ideas such as charms and incantations. Chachamim did not uproot things that were not forbidden on account of idolatry or Darchei ha'Emori. Some matters were so ingrained in people that they would physically weaken or strengthen them. E.g., it was taught 'if one is particular about it, it is particular to harm him.' People were so concerned for Zugos that Chachamim needed to give reasons why there is no concern regarding the four cups.

Question: A tradition from Sinai teaches about Zugos!

Answer: (Shemiras ha'Guf veha'Nefesh 28 DH v'Acharei) Sometimes 'a tradition from Sinai' merely means a matter as clear as a tradition from Sinai, but it is really mid'Rabanan (Ri, brought in Rosh Hilchos Mikva'os (printed after Nidah), Siman 1).

Poskim

Tur (OC 170): One should not drink an even number of cups, for this is dangerous.

Question (Beis Yosef DH Garsinan b'Perek): Tosfos says that the Ru'ach Ra'ah is not frequent among us! If the Tur argues, he should have brought the distinctions in the Gemara about when we are concerned!

Answer (Drishah 2): The Tur does not come to teach that we are concerned for Zugos today. Rather, the Gemara forbids drinking two cups at a meal, and bringing two cups before Birkas ha'Mazon. The latter is forbidden for it looks like gluttony; the Tur teaches that the former was not forbidden because of gluttony.

Yam Shel Shlomo (Chulin 8:12): Ruchos such as Zugos are not common nowadays. Nevertheless, one should be concerned for Zugos for it makes one vulnerable to witchcraft.

The Chazon Ish was careful to avoid Zugos (cited in Shemiras ha'Guf veha'Nefesh 28).

Note: Perhaps this is because an important person is different (Gemara; Rashi - Mazikim make every effort to damage him).