I think you can safely rule out Flemish. With an adult weight in excess of 13 lbs, a Flemish Giant would not weigh only 4 lbs at 8 months of age. Besides, there really isn't anything about this rabbit that suggests Flemish.
IDK why Rex is coming up, either, since she clearly does not have a Rex coat.
This rabbit is Chinchilla colored. The smallest of the Chinchilla breeds, the Standard Chin, matures around 6 lbs. That's a bit bigger than this rabbit, but it's a lot likelier than Flemish as the source of that color. Of course, Mini Rex come in Chinchilla, though it's not a real common color for them and, as I said, this rabbit doesn't have a Rex coat.