Some say that they were 66 and that the missing 4 are: Yocheved, Yosef and his 2 sons. How about Osnas (Dina's daughter, Yosef's wife). If Yosef and his children were counted why not Osnas?
A¹ according to the Tur on Torah you can say Asnas was the 70th. A² according to chachmei Tzarfas and Chizkuni in parshas Emor by the ger you can say since Asnas father was Shchem therefore she wasn't Jewish or related to Dina. A³ according to the Moshav Zikeinim she wouldn't be listed because the count doesn't include the wives of the children of Yakov. A⁴ according to the Taima Dikra that Serach bas Asher was really the daughter of Ashers wife and reason she's counted is because since Asher raised her it's as if he gave birth to her. Can say since Potifar raised Asnas she's considered his daughter and not Dinas. A⁵ according to the Pirkei Drav Eliezer Asnas went to Egypt just to marry Yosef, so one can say she's not counted since didn't go to Egypt because of the shibud or slavery.
I think she is counted, just not expressly for the same reason she is never expressly described as Dinah's daughter--the story of her conception is an embarrassment to her mother.
We all know the question that if you count the names of Yaakov's family there are only 69 names. There are two primary answers to this question in the Rishonim: 1) Rashi, quoting the Gemara, says that the 70th is Yocheved; 2) the Ramban and Ibn Ezra say that the 70th is Yakov himself. Each of this answers has weaknesses.
1. Yocheved as the 70th
A) If Yocheved is the 70th, why not say her name expressly? Pesukim elsewhere expressly say she was the daughter of Levi (Bamidbar 26:59; Shemos 6:20).
B) If Yocheved is the 70th, she gave birth to Miriam when she was 125, Aharon at 127, and Moshe at 130. Why is this incredible miracle not mentioned in the posuk?
2. Yaakov as the 70th
A) Yaakov makes sense if one only looks at the end of the parsha because he is not יֹצְאֵ֣י יְרֵכ֔וֹ and therefore not part of the 66 that went down (Bereishis 46:26) but is part of כָּל־הַנֶּ֧פֶשׁ לְבֵֽית־יַעֲקֹ֛ב (Bereishis 46:27). But that count isn't the only issue. By the count of the children of Leah it says:
אֵ֣לֶּה ׀ בְּנֵ֣י לֵאָ֗ה אֲשֶׁ֨ר יָֽלְדָ֤ה לְיַעֲקֹב֙ בְּפַדַּ֣ן אֲרָ֔ם וְאֵ֖ת דִּינָ֣ה בִתּ֑וֹ כָּל־נֶ֧פֶשׁ בָּנָ֛יו וּבְנוֹתָ֖יו שְׁלֹשִׁ֥ים וְשָׁלֹֽשׁ
These are the children of Leah that she bore to Yaakov in Padan Aram and with Dinah her daughter all the souls of his sons and daughters, 33. (Bereishis 46:15)
But if you count the names of Leah's children there are only 32, not 33. Yocheved fits as she was a daughter of Levi. But how can Yaakov himself be counted as one of his children from Leah?
So we need another option that fits with all of the pesukim.
3. Osnas as the 70th
Counting Osnas as the 70th member of Yaakov's family explains all of the pesukim. Osnas is not mentioned expressly because of the embarrassing circumstances of her conception and adoption. She is counted among the children of Leah in Bereshis 46:15 because she is daughter of Dina. She is not counted among the 66 that went down in Bereishis 46:26 because she was already in Egypt. But she is counted in Bereishis 46:27 as part of the 70 just like her husband and her husband, Yosef, and her sons.
Not ignoring the previous answers, but just adding a thought al pi drush.
The sefer Chesed Shmuel (I think it is) says that it was only permissible for Yosef to marry Osnas because he was in chutz la'aretz (using the same logic that it was only permissible for Yaakov to marry sisters because he was in chutz la'aretz.
Therefore you could not possibly say that Osnas was one of the 70 who came to Mitzrayim, because to the extent she was shayach to Eretz Yisrael she was forbidden to Yosef.