וְד֥וֹר רְבִיעִ֖י יָשׁ֣וּבוּ הֵ֑נָּה כִּ֧י לֹא־שָׁלֵ֛ם עֲוֺ֥ן הָאֱמֹרִ֖י עַד־הֵֽנָּה׃
(Modified Sefria.org translation)
'And in the fourth generation they shall come back here; for the sin the Amorite is not yet full.’
If we account for the sin of the spies and the fact that the generation of those that were age 20+ that left Egypt, we find that because the people believed the report of the 10 spies, G-d said that the next generation would be the ones to inherit the land in Cana'an.
So, when G-d mentioned to Avram the 4th generation, does this number include or exclude the fact that the "original" generation would have entered the land had they not sinned by believing the spies' report?