Why would you want to make Bron a 4? He's already a mismatch for everyone at his natural position of SF. Put him at the 4 and he has to start banging for boards and defending post scorers. As for getting to play against opposing 4s on D, opponents would just switch their 4s onto the far less dangerous Artest and happily accept his moderate match-up advantage to have Bron get smacked around among the big men.
And I'm not sure a contending team wants Artest as their starting 4 if you want to play it that way.
Lebron won't guard the 4. Hell against Orlando he barely guarded the 3 as the Cavs hid him against the 2 most of the time.
Put it this way, if the cavs and C's are playing in the playoffs, then they will go big with Williams, Artest, Lebron, Varejao, and Shaq and Lebron will guard Ray Allen or Rajon Rondo. If they are playing the Magic they will go small with Williams, West, Artest, Lebron, and Shaq and Lebron will have Carter or Nelson.