1) What are the proper ablution rituals to perform after a man or woman has sex and after a woman stops menstruating? Also, are there certain Dua' that one should recite in these circumstances?2) I've heard of many stories (possible Ahadith?) saying that if you say a certain Dua' or Surah a certain number of times then you will be rewarded in a certain manner. Are these generally substantiated in authentic Hadith or have they been misunderstood? For example, if you say al-Fatiha 4 times, Surah al-Ikhlas 3 times, Surah al-Kafiroon 4 times, La Hawla Wala Kuwata Illa Billah 7 times, and La Ilaha Illa Allah Wahdahu 10 times at night before you go to sleep, then all of the minor sins you committed that day will be forgiven.

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

1) Taking a bath due to major ritual impurity or menstruation can be done in two ways. For details, consult Fatwa No. 82245.
As for Dua' (supplication) while taking a bath, there is no any lawful approved Dua' that was enacted by the Prophet (Sallallahu Alaihi wa Sallam).
If there were any narration in this concern, it would be narrated by the Prophet's (Sallallahu Alaihi wa Sallam) wives or his companions.
2) Allah Says (interpretation of meaning): {O you who believe! Remember Allâh with much remembrance.}[33:41] He also Says: {Therefore remember Me (by praying, glorifying, etc.). I will remember you…}[2:152].
These two verses state that a Muslim is advised to observe Zikr of Allah without a fixed number. But, the Sunnah legalizes repetition of some Surahs and verses, forms of Zikr and Dua' in fixed numbers, and mentions the reward for each.
For example, the Prophet (Sallallahu Alaihi wa Sallam) said: "He who says daily in the morning: In the name of Allah, by Whose name nothing in the earth or in the Heaven can do any harm, and He is All-Hearing, All-Knowing, three times, nothing will harm him until - evening". [Ibn Majah]
Also the Prophet (Sallallahu Alaihi wa Sallam) said: "Reciting the Surah of al-Ikhlas equals one third of recitation of the Qur'an in reward". [Reported by Imam Muslim]
In a third Hadith he said: "Reciting al-Ikhlas and the last two Surahs of Qur'an daily in the evening and morning three times suffices (a reciter and protects him) from every (bad) thing". Al-Tirmizi, al-Nasae', Abu Dawood and recorded by al-Albani in al-Jama' al-Saghir under No. 1534.
For more forms of Dua' and Zikr and good deeds, consult sound, elaborate books in this issue, we advise you to seek such an information through trustworthy scholars in your country and to avoid weak forms, especially in issues related to knowledge.
Here are some good books:
1) al-Nasih Fil Azkar, by Muhammad Ibn Ismael al-Muqadam.
2) Sahih al-Kalim al-Tayeb by al-Sheikh al-Albani.
3) Riyad al-Saliheen, by al-Nawawi.
N.B:- The last two books are available in English.
As for your question about reciting al-Fatiha four times, there is nothing like this in the approved books of Sunnah.
Thus, we advise you to adhere only to forms of Dua' and Zikr that are approved and confirmed in the Sunnah.
Allah knows best.