Vayigash #4: Are Reuven's Children Tribbles?

My wife pointed out to me an interesting detail I overlooked. When Reuven tries to convince Yaakov to let them take Binyamin in Miketz, he mentions his two sons. Yet, in the counting in Vayigash, we are told of Reuven's *four* sons. It is possible but unlikely for him to have an additional two sons in the intervening period. The relevant psukim:

Bereishit 42:37:

וַיֹּאמֶר רְאוּבֵן, אֶל-אָבִיו לֵאמֹר, אֶת-שְׁנֵי בָנַי תָּמִית, אִם-לֹא אֲבִיאֶנּוּ אֵלֶיךָ; תְּנָה אֹתוֹ עַל-יָדִי, וַאֲנִי אֲשִׁיבֶנּוּ אֵלֶיךָ.
"And Reuben spoke unto his father, saying: 'Thou shalt slay my two sons, if I bring him not to thee; deliver him into my hand, and I will bring him back to thee.' "

And in the counting, in Bereishit 46:9

וּבְנֵי, רְאוּבֵן--חֲנוֹךְ וּפַלּוּא, וְחֶצְרֹן וְכַרְמִי.
"And the sons of Reuben: Hanoch, and Pallu, and Hezron, and Carmi."

Again, this may mean that וְאֵלֶּה שְׁמוֹת בְּנֵי-יִשְׂרָאֵל הַבָּאִים מִצְרַיְמָה might mean a count of the initial group of Benei Yisrael when they entered into Egypt, as compared with the count of Benei Yisrael when they left.