When you cross over a speaker to a sub, some bass still makes it through to the speaker. So if you listen very loud, IM distortion will still result from the bass. Of course it's not only bass that causes IM distortion. Mids will as well when pushed hard. v4 mids can take more than earlier versions.

Perhaps though, the tweeter is being pushed too hard. Pre-v4 tweeters can't take as much tweeting. Or, the receiver is distorting.