(1, 2) If semen was discharged due to masturbation then the fast will break and one has to make it up.
(3) No, the fast will not break.
(4) Do not gargle while fasting, otherwise it is feared that it may cause water reaching to inside throat. If the water did not reach inside throat the fast shall remain intact.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband