1,is about a double attend suicide(or they think),the happen between a teacher nurse and student in tifon night the both fall,and the school think it was attend of suicide because his forbidden love,but the truth is the first time the meet,but after the incident they coming to meet,the boy talk to the teacher about his love with his older brother and something happen so the was the reason his want too die ,and too the hands teacher feel his brother and raped the teacher,so the relationship continues.

2.Is about and detective or polices(i think)the live with his ex-girlfriend son because his mother found other man ,and later enter too a sexual relationship,but later his found his muder mother...not really remember.

3.is about incest,the older cousin/uncle teacher at the college of the other boy,only some year apart,the younger boy always love his older cousin,and now the live(i think)as lover,but before him his older cousin live with other over the moved overseas and the new lover feel insecure about the thing the other one leaf ,the part i must remember is like top of cup,,i don;t really remember .

4.is only a scene i remember,,,the having sex and his run to the bathroom to pee ...and later pass super hot scene...

Hmmm...even if the description doesn´t really match, nr 2 sounds like the first story of Boku no Shiru Anata no Hanashi

#1 sounds like Kimi no Te wa Boku no Mono, the first story