You keep switching gears on me. At least from my own point of view. Making it difficult for understanding.
You mention grafting. Again this to me is the kingdom of Messiah which is the heavenly kingdom not the earthly. We are not graft into the earthly inheritance with the natural sons of the fourth generation. Those which are cut off, are those which are made void of the earthly inheritance. They can be graffed into the heavenly at anytime they turn to Christ.
First of all: the Judaism of first century as well as scripture, was Levitical, temple ministration. What is called Judaism today is not the same. They are not equal.
Second of all, Gentiles need not convert to either the temple ministration of Levi, nor are Gentiles heirs of the earthly inheritance, with the 4th generation. Nor was Abraham an heir with them. Take note of verse 15..
Gen 15:13 And he said unto Abram, Know of a surety that thy seed shall be a stranger in a land that is not theirs, and shall serve them; and they shall afflict them four hundred years;
14 And also that nation, whom they shall serve, will I judge: and afterward shall they come out with great substance.
15 And thou shalt go to thy fathers in peace; thou shalt be buried in a good old age.
16 But in
the fourth generation they shall come hither again: for the iniquity of the Amorites is not yet full.
Abraham is told he would die. This covenant is made concerning the
fourth generation of his seed.
18 In the same day the LORD made a covenant with Abram, saying,
Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
19 The Kenites, and the Kenizzites, and the Kadmonites,
20 And the Hittites, and the Perizzites, and the Rephaims,
21 And the Amorites, and the Canaanites, and the Girgashites, and the Jebusites.
Acts tells us this as well
Acts 7:2 And he said, Men, brethren, and fathers, hearken; The God of glory appeared unto our father Abraham, when he was in Mesopotamia, before he dwelt in Charran,
3 And said unto him, Get thee out of thy country, and from thy kindred, and come into the land which I shall shew thee.
4 Then came he out of the land of the Chaldaeans, and dwelt in Charran: and from thence, when his father was dead, he removed him into this land, wherein ye now dwell.
5 And
he gave him none inheritance in it, no, not so much as to set his foot on: yet he promised that he would give it to him for a possession, and to his seed after him, when as yet he had no child.
6 And God spake on this wise, That his seed should sojourn in a strange land; and that they should bring them into bondage, and entreat them evil four hundred years.
7 And the nation to whom they shall be in bondage will I judge, said God: and after that shall they come forth, and serve me in this place.
As the book of Hebrews also tells us
Heb 11:13 These
all died in faith,
not having received the promises, but having seen them afar off, and were persuaded of them, and embraced them, and confessed that they were
strangers and pilgrims on the earth.
Bottom line for me. I do not need to become an heir of the earthly kingdom of this life God promised concerning the fourth generation of his seed. The kingdom inherited in Christ is much more excellent.