1:
The first part of our present mishnah reads almost the same as the previous mishnah. However, the subject is slightly different than the one dealt with by the previous mishnah. In the previous mishnah the wine which was made by the Jew for his non-Jewish employer belonged to the non-Jew, even though he wanted to sell his wine also to his Jewish customers. In our present mishnah the situation is slightly different.

2:
In our present mishnah the Jew who prepared the wine for his non-Jewish employer wants to purchase the wine from the non-Jew and to sell it himself - presumably to Jews. The ruling of our mishnah is quite clear; however, because it omits clarifying subjects for several of the verbs it requires a clearer explanation.

3:
If the non-Jewish employer gives the Jew a receipt for money which the Jew paid for the wine, the wine is kosher: it is not yeyn nesekh even though it originally belonged to a non-Jew. The reason is clear: the wine was prepared by a Jew and the non-Jew wanted the wine to be salable to Jews. Knowing Jewish law he was careful not to touch the wine - as we learned in the previous mishnah. However, in the previous mishnah we learned that under certain circumstances it would be necessary to appoint a Jew to watch over the wine until it was sold. In our present case this is not necessary: the wine is permitted because the non-Jew has already sold it to the Jew; thus it is no longer his and he will be wary of touching someone else's property. This holds true even if the Jew leaves the wine still in the premises of the non-Jew.

4:
However, if the Jew wants to remove the wine from the premises of the non-Jew before he has paid for it and the non-Jew will not release the wine until payment has been made the sages declare the wine to be yeyn nesekh. The reason is clear: the non-Jew continues to look upon the wine as belonging to him until payment has been made and therefore will have no compunctions about touching his own property in the absence of the Jew.

5:
However, the situation is not as simple as our mishnah describes it. In the Tosefta [AZ 8:3] we read:

So it seems that the Tosefta adds a requirement that was not mentioned in our mishnah: the wine is not yeyn nesekh as long as it is capped or sealed - because then it would be obvious whether or not the non-Jew had opened it.

6:
This, at long last, concludes our study of Chapter 4. God willing, in our next shiur we shall begin our study of the 5th and last chapter of this tractate.

In AZ 059 we learned:

I explained:

In the case of the baker, we are permitted to assist him in transporting his loaves from the bakery to the shop where they will be sold because the damage has already been done: the bread is ritually impure and there is no way that we can make it pure now.

Amnon Ron'el writes:

Helping a baker who works while ritually impure to transport loaves of bread for sale in a shop is assisting him in deriving benefit from impure produce and encouraging him to continue thus. I understand the mishnah to be prohibiting someone who works while ritually impure from touching food: "We do not tread or harvest grapes with a Jew who works while ritually impure ... we do not knead or roll dough with a baker who works while ritually impure"; but does permit him to carry packaged food: in a cask, a utensil or a cart.

I respond:

It's not that simple. Even if the food is packaged the impurity is still imparted to it. (The rules concerning ritual purity and impurity are complex. We studied them in detail when we studied Tractate Yadayyim.)

What we must understand is that the problem of ritual impurity only applies to priests and those lay people who paid particular attention to these complex rules and regulations. When we studied tractate Yadayyim we mentioned that in Tannaïtic times society was divided, as it were, between the ĥaver and the am ha-aretz. I wrote:

Thus the bread which is the subject of the mishnah could be sold to ordinary folk who did not observe the minutiae of these laws. Priests and sages would have to be careful and, presumably, only bought from reliable sources.