Why can’t Winston prove Oceania was at war with Eurasia 4 years ago?

Winston can't prove it because history has been changed and minds manipulated... Winston on the other hand was not completely under control, thus he knew it was only four years since Oceania had been at war with Eastasia and in alliance with Eurasia, but he couldn't prove it.

Actually, as Winston well knew, it was only four years since Oceania had been at war with Eastasia and in alliance with Eurasia. But that was merely a piece of furtive knowledge which he happened to possess because his memory was not satisfactorily under control.