As a starting point, lets just say that anyone, even Joe Mauer, hitting over .400 is highly unlikely. So when we ask the question, the obvious, simple, answer is no. But we aren't really talking about something that is likely. We are asking a question about whether the highly improbable is still possible.
If we assume that Mauer gets 500 at bats he will need 200 hits to hit .400. He currently has 139 hits in 363 at bats. so he would have to hit 61 for 137 (.446) to make the 200 for 500 mark and hit .400. So far this year, Mauer has yet to sustain anything like that number for a full calendar month. He hit .414 when he came back in May. On the other hand, he is hitting .485 so far this month (as of August 19th). So while the idea remains highly improbable, he probably is closer to it than he was back in June when people were already talking about it with four months still to go in the season.