[4a] is ambiguous in a way [4b] is not. Its second conjunct can be construed as saying either that Juan beats his own donkey or that Juan beats Pedro’s donkey. The second conjunct of [4b], by contrast, can only be interpreted as saying that Juan beats his own donkey. If the possessive pronoun in the first conjunct of [4a] can be either referential or a bound variable, the contrast is readily explained.