Guaranteed entry to Olam HaBah

Question

I learned from a rabbi that, if you live to see the 4th generation, your great-grandchildren, you automatically get into Olam HaBah. I love this idea. I know that all of Israel gets in - but this differentiates that group somehow. I cannot find the source. Does anyone know it?

Adding this, not sure it's good enough of an answer but would love your feedback:

I found this midrash that might work? this section from Midrash Tehillim 119 almost gives it by the transitive property (If you keep Torah God will 1 - keep your soul and 2 - allow you to see grandchildren.)

What is "take care"? God said, "If you keep the Torah, I will keep your soul" (Talmud, Shabbat 127a). Therefore it is said, "Take care and keep your soul diligently all the days of your life" (Deuteronomy 4:9). What are "all the days of your life"? If you do the Torah all the days of your life, you will merit to see children and grandchildren. Therefore it is said, "And you shall teach them to your children and your grandchildren" (Deuteronomy 4:9). And if you see children and grandchildren, then there will be peace, as it is said, "May you see your children's children! Peace be upon Israel!" (Psalms 128:6).

Answer 1

The Leket Yosher says it's customary to appoint the great-grandfather sandek at a bris because there's a saying that one who is sandek for his great-grandson will not see gehinnom:

מי שיש לו בן למול ואינו רוצה להיות בעל ברית, ורוצה לכבד אחרים משום הבאת שלום ומשום דיש בו כבוד הבריות, חייב ליתן לאביו קודם אביו זקנו להיות בעל ברית. אבל אם הוא ברצון אביו יוכל לתן לזקנו, והמנהג ליתן לזקנו משום שהעולם אומרים מי שהוא בעל ברית לבן נינו אינו רואה פני גהינם

Answer 2

I have never heard this before, but it is reminiscent of an idea in the Torah rules of reincarnation.

Based on the pasukim in Shemot 20:5-6

לֹא־תִשְׁתַּחֲוֶה לָהֶם וְלֹא תׇעׇבְדֵם כִּי אָנֹכִי ה אֱלֹקֶיךָ אֵל קַנָּא פֹּקֵד עֲוֺן אָבֹת עַל־בָּנִים עַל־שִׁלֵּשִׁים וְעַל־רִבֵּעִים לְשֹׂנְאָי׃

You shall not bow down to them, nor serve them: for I, Hashem your God am a jealous God, punishing the iniquity of the fathers upon the children unto the third and fourth generation of those that hate Me;

וְעֹשֶׂה חֶסֶד לַאֲלָפִים לְאֹהֲבַי וּלְשֹׁמְרֵי מִצְוֺתָי׃        

but showing mercy to thousands of generations of those that love Me, and keep My commandments.

The pshat of this is talking about familial generations, but the secret here is talking about one's reincarnations. The Arizal teaches us, in conjunction with Iyov 33:29, Amos 2:6 and Sefer HaTikkunim 69 that this is coming to teach us that if a person is reincinarnated a 4th time, they are guaranteed Olam Haba. See Shaar HaGilgulim 4:6.

This is esoteric Torah, but the very short and basic explanation is that if a person comes back 3 times and doesn't accomplish any rectification in any of those 3, then he will not be reincarnated again, but will be cut off, ch'v.

However, if in those 3 he completes even one rectification, not only will he now be zoche to be reincarnated up to 1000 times, but Hashem guarantees that He will achieve complete rectification.