Who knows four?
Please cite/link your sources, if possible. After Shabbat, I will:
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Four men died without sins:
Binyamin, Amram, Yishai, Kil'av.
... levels on which to understand the Torah: פשט- plain meaning, רמז- hints, דרש- exegesis, סוד- secret, hidden, underlying essential meaning
The number of columns (turim) of stones on the Choshen Mishpat. (Exodus 28:17)
Ironically enough, Choshen Mishpat is one of the Arba'a Turim. Which brings another answer: the number of sections in the Arba'a Turim and Shulchan Aruch.
Four sons before Leah gave praise/thanks to Hashem (Yehuda). Four are the number of wives (mates) that Yaakov had. Twelve divided by Four equals Three sons per wife. Thus when Yehuda was born, Leah realized she got even more than her "fair share" and gave a special thanks.
the most popular number during the Passover Sedar. Four questions. Four sons. Four cups of wine.
4 holy cities, Yerushalayim, Chevron, Teveria, Tzefas
4 shades of white that constitute tzara'at. (Snow, plaster, eggshell and wool)
I know we all like to provide out-of-the-box answers, but I'm disappointed that I have to be the one to give this answer, and nearly two years after the question was asked - 4 Imahoth (Matriarchs).
We take 4 Minim on Succos: Lulav, Esrog, Hadas, Arava (Leviticus 23:40)
Four couples buried in Mi'aras HaMachpeilah in Kiryas Arba Hi Hevron.
Four are the levels of the separative trop.
The kabbalistic worlds: atzilut, yetzira, beriya, and asiyah.
Four rivers emgered out of Gan Edhen "וְנָהָר יֹצֵא מֵעֵדֶן, לְהַשְׁקוֹת אֶת-הַגָּן; וּמִשָּׁם, יִפָּרֵד, וְהָיָה, לְאַרְבָּעָה רָאשִׁים"
Four kingdoms allied and won the first great war " אֵת כְּדָרְלָעֹמֶר מֶלֶךְ עֵילָם, וְתִדְעָל מֶלֶךְ גּוֹיִם, וְאַמְרָפֶל מֶלֶךְ שִׁנְעָר, וְאַרְיוֹךְ מֶלֶךְ אֶלָּסָר--אַרְבָּעָה מְלָכִים, אֶת-הַחֲמִשָּׁה."
Four corners require SiSith "גְּדִלִים, תַּעֲשֶׂה-לָּךְ, עַל-אַרְבַּע כַּנְפוֹת כְּסוּתְךָ, אֲשֶׁר תְּכַסֶּה-בָּהּ"
We Daven four Amidos (colloquially known as Shemonah Esreis) on Shabbos, Yom Tov, and Rosh Chodesh
Starting from the Shabbos before Rosh Chodesh Adar - or Adar2 in a leap year - we read what is known as the Daled Parshiyos - 4 special readings for the Maftir. Parshas Shekalim, Parshas Zachor, Parshas Parah, and Parshas HaChodesh.
There are four types of death by a Beis Din - Stoning, Burning, Beheading and Strangulation. (Sanhedrin 49b)
They carried out the four death sentences against him (Bilaam). - Sanhedrin 106b
Four are the levels of tum'ah applicable to non-sacred foods.
Pirkei Avos Ch. 5:
4 times when pestilence increases - 4th, 7th, and year after shemitta, and after sukkos (12)
4 types of people - what's mine is mine etc... (13)
4 types of mindsets - angers easily cools off easily, angers slowly cools off slowly, angers slowly cools off easily, angers easily cools off slowly (14)
4 types of students - Understand quickly and forgets quickly, understands with difficulty and forgets with difficulty, understands quickly and forgets with difficulty, understands with difficulty and forgets quickly (15)
4 types of tzeddaka givers - gives himself and wants others to give etc... (16)
4 types of people who go to the Beis Medrash - goes and doesn't study, studies and doesn't go, goes and studies, doesn't go and doesn't study (17)
4 types of people who sit before chachamim - sponge, funnel, strainer, sieve (18)
four letter of the tetragrammaton Y-H-V-H which is ultimately the source of most things four you will find in the torah. see shaarei kedusha.
There are four shomerim - שומר חינם, שואל, נושא שכר, ושוכר. (Bava Metzia 93a)
Moshiach has four names. Menachem, Shilo, Yinon, and Chanina which spells Moshiach
Four expressions of redemption from Egypt.
On Rosh Chodesh we have four Aliyos to the Torah.
Four is the number of cities King Nimrod sacrificed when he abandoned the cause of the Tower of Babel: Babel, Erech, Achad, and Kaleh.
Four is also the number of cities he was able to build as a reward once he got to Ashur: Ninveh, Rehoboth Ir, Kalah, and the great city, Resen.
(Gen 10:10-12, Targum Pseudo Jonathan ibid)
There are four verses in Megillas Esther which the listeners read out loud.