Gemara

Question: May one be Yotzei [the four cups] with Konditon (wine with honey and peppers mixed in)?

Answer: Yes, for Bar Kapara taught that Konditon is like wine.

Question: May one be Yotzei with diluted wine?

Answer: R. Chiya taught that one is, as long as it has the taste and appearance or wine.

(R. Yirmiyah): The Mitzvah [l'Chatchilah] is with red wine - "Al Tir'eh Yayin Ki Yis'adam..."

(Beraisa): Mevushal wine is like seasoned wine.

Question: May one be Yotzei with Mevushal wine?

Answer (R. Yona): Yes.

This is consistent with R. Yonah's practice;

When R. Yonah would drink the four cups on Pesach, his head would hurt until Sukos.

Nedarim 49b - R. Yehudah: The only wine I drink is for Kidush, Havdalah, and the four cups on Pesach, and then my head hurts from Pesach until Shavuos.

Rishonim

Teshuvas ha'Rashba (1:238): If one does not drink wine the entire year because it harms him, or he hates it, he must force himself to drink. [Normally] R. Yehudah did not drink wine, but he drank the four cups, and his head hurt from Pesach until Shavuos. R. Yonah drank the four cups, and his head hurt until Sukos. One who does not have wine may make the Seder on bread, but we do not learn about what is possible from what is impossible. Also, it is possible with Konditon and Mevushal wine, like the Yerushalmi says.

Korban ha'Edah (8b DH v'Chazik): Because R. Yonah's head would hurt until Sukos, he was Machshir Mevushal wine, which does not harm so much.

Rashbam (Pesachim 108b DH Al): We learn from "Al Tir'eh Yayin Ki Yis'adam..." that wine requires the appearance [of wine]. It also requires the taste [of wine], like the verse warns not to get drunk.

Poskim

Shulchan Aruch (OC 204:5): Our wines are not so strong. Even if one puts three measures of water on dregs, and four measures exude, one does not bless Borei Peri ha'Gafen. It seems that it depends on how they mix wine in the region.

Rema: As long as there are not six measures of water per measure of wine, for then the wine is surely Batel.

Kaf ha'Chayim (5): Olas Tamid says "nowadays that the custom is not to mix wine at all with water, for our wines are very weak. If a little water was mixed in, even less than the amount of wine, one does not bless Borei Peri ha'Gafen. In practice, this requires investigation." Eliyahu Rabah says that this is why the Levush omitted the Rema's opinion. Eshel Avraham says that our wines not strong, so if there is a majority of water was mixed in, one does not bless Borei Peri ha'Gafen. It seems that if they are equal, one blesses Borei Peri ha'Gafen. On should be careful not to mix more than this, even in pressed circumstances when one has only a little wine.

Mishnah Berurah (272:16): Bechor Shor derives from the Rema that one part of wine is Batel in six parts of water. One does not bless on it Borei Peri ha'Gafen. Therefore, in raisin wine, the raisins must be at least a sixth of the volume of the water.

Bi'ur Halachah (DH Mekadshin): Mishkenos Yakov (96) considers only the moisture that comes out of the raisins (not their entire volume), but it must be at least a quarter of the volume of the water [with the added moisture from the raisins], like Rava taught, that a proper wine must be able to tolerate dilution with three times as much as water. One who is able should get a good wine for Kidush to fulfill all concerns and fulfill the Mitzvah in the ideal way. However, since many Poskim hold like the Rosh and Tur, brought in Rema YD 123:8, that the taste of wine suffices, it suffices to be stringent like Bechor Shor. Many Gedolim bring his opinion.

Shulchan Aruch (272:8): One may say Kidush over Mevushal wine, and on wine with honey.

Mishnah Berurah (272:19): The two opinions about Mevushal wine argue about whether or not it changed for the worse.

Shulchan Aruch (ibid.): Some forbid both of these.

Mishnah Berurah (21): The Rambam forbids even if one added any amount of honey. Those who hold like the latter opinion should be concerned for this, and not add any honey or sugar.

Rema: The custom is to be Mekadesh on it, even if he has other wine, just it is not as good as the Mevushal wine or the wine with honey.

Mishnah Berurah (22): Therefore, one may cook raisins, strain off the liquid and be Mekadesh over it. Some are stringent not to consider it wine until it bubbles three days after the raisins were cooked.

Mishnah Berurah (23): If the wines are of equal quality, one should be concerned for the stringent opinion not to be Mekadesh on it. Regarding Berachah, we clearly rule that one says Borei Peri ha'Gafen.

Shulchan Aruch (472:10): One who does not drink wine because it harms him, or he hates it, must force himself to drink, to fulfill the Mitzvah of four cups.

Magen Avraham (12): It seems that one may dilute the wine, as long as it is proper for Kidush, like in Siman 272.

Mishnah Berurah (35): It harms him, i.e. he is pained to drink it, and his head hurts. This does not include when he will be bedridden due to this.

Kaf ha'Chayim (69): See what I wrote in 473:88 [that if a person will get sick for a day or two due to eating Maror, he is exempt. It is also forbidden, due to harming oneself - Besamim Rosh 94.]

Kaf ha'Chayim (71): One who is in pain is exempt from Sukah. There is different, for the Mitzvah is to dwell in the Sukah like he dwells in his house. For others Mitzvos, no verse teaches that pain exempts. Chachamim enacted like Torah law.

Kaf ha'Chayim (72): One should force himself to drink four full cups, and eat a k'Zayis of Matzas Mitzvah and Afikoman, even if this is difficult. Through this, he will not need to eat or drink bitter medicines to heal the body (Orchos Yosher).

Kaf ha'Chayim (73): The four cups come to atone for the four Misos Beis Din, if one was liable for one of them (Iyun Yakov).

Mishnah Berurah (36): On Shabbos and Yom Tov, one can be Yotzei through hearing Kidush from someone else. Here, the Chiyuv to drink is on everyone.

Mishnah Berurah (37): One can take also raisin wine or Chemar Medinah (an esteemed drink of the region).

Shulchan Aruch (11): It is a Mitzvah to pursue red wine [unless the white wine is superior].

Beis Yosef (DH v'Isa): The Ramban holds that if one was Mekadesh on white wine, even b'Di'eved he was not Yotzei. The Tur disagrees. He permits Kidush on it even l'Chatchilah, for one of the four cups is Kidush, and he says that if the white wine is superior, it has precedence. The Roke'ach agrees. The Rif, Rambam and Rosh are unlike the Ramban.

Taz (9): The Tur was particular about red wine for the four cups, but not for Kidush. Red also commemorates the blood. Paro used to slaughter Yisre'elim [and bathe in their blood]. Nowadays, people refrain from red wine due to false accusations [that Yisre'elim drink blood of Nochrim].

Kaf ha'Chayim (78): Eshel Avraham says that red commemorates the plague of blood, just like Charoses commemorates the mortar. Also, it alludes to the future [when Hash-m will punish Edom] - "Mi Zeh Ba me'Edom... Madu'a Edom Malbushecha."

Gra (DH Im): Even R. Yehudah, who requires that it be red, requires taste and appearance (Pesachim 108b). See the Rashbam.

Kaf ha'Chayim (79): The Rashbatz is concerned for the Ramban, who forbids white wine without any redness for Kidush or the four cups. However, he (Rashbatz) permits it l'Chatchilah for Kidush [on any other Shabbos or Yom Tov] and Havdalah (Aruch ha'Shulchan).

Shulchan Aruch (12): One is Yotzei with Mevushal wine and Konditon.

Beis Yosef (DH v'Yayin): Rav Hai Gaon says that one is not Yotzei with Mevushal wine. Most Poskim say that one blesses on it Borei Peri ha'Gafen (Siman 272).

Mishnah Berurah (39): It is better to take non-Mevushal wine, unless the Mevushal wine is better.

Mishnah Berurah (40): See what we wrote in 272:8. The same applies here.