Return - The repentance of Israel described in Jeremiah 3:21-25 was a hope, and not a reality. The return, literally, would be their restoration to their land; spiritually, their abandoning their sins.
Jeremiah 4:1-2 should be translated as follows:
If thou wouldst return, O Israel, saith Yahweh.
Unto Me thou shalt return:
And if thou wouldst remove thy abominations from before Me,
And not wander to and fro,
But wouldst swear truly, uprightly; and justly
By the living Yahweh;
Then shall the pagan bless themselves ... -
In him - In Yahweh. Two great truths are taught in this verse;
(1) that the Gentiles were to be members of the Church of the Messiah;
(2) that Israel’s special office was to be God’s mediator in this great work.
Thus, Jeremiah is in exact accord with the evangelical teaching of Isaiah.