(a) Beis Shamai permits the owner to exchange an entire Sela of Ma'aser Sheni money for Perutos in Yerushalayim (to cover his day to day expenses) - Beis Hillel permit him to exchange only half (see Tos, Yom-Tov) ...
(b) ... in case he uses up only some of them and is forced to leave the others in Yerushalayim until the following Yom-Tov, by which time they will have gone bad (as copper coins tend to do).
(c) The 'Danin Lifnei Chachamim' - ben Azai, ben Zoma and Chanan ha'Mitzri (see Tos. Yom-Tov).
(d) Rebbi Akiva and Rebbi Tarfon are even more stringent than 'Danin Lifnei Chachamim'. Rebbi Akiva permits him to change only a quarter of a Dinar into Perutos at a time (one sixteenth of a Dinar). When Rebbi Tarfon says four Aspri of silver - he means that one is permitted to exchange four Aspri per Dinar to one of copper, which is the equivalent of one twentieth of a Sela.
(a) Shamai does not permit exchanging a Sela for Perutos at all - because he is afraid that the owner may forget that are Ma'aser, and treat them as Chulin coins.
(b) When he needs to purchase food - he leaves the Sela with storekeeper deducting from it with each purchase until it is finished.
(c) The Halachah is - like the Tana Kama.
(a) The problem our Mishnah discusses with regard to someone with Ma'aser Sheni wine outside Yerushalayim and some of whose sons are Tamei is - there where he wants them all from the same earthenware vessel containing wine which includes Tevel of Ma'aser Sheni.
(b) What he must do is - place a Sela coin before they begin to drink and declare all the wine that his Tahor sons drink Ma'aser Sheni retroactively (see Tos. Yom-Tov and Tiferes Yisrael).