I'll go with Leader, too. They feel more like enablers. That can be very rewarding to play, but I think it would be strange if the players would feel as if it was them setting up the enemy so the DMPC can come rescue them.
Defender is always in the face of the enemy and taking hits. A DMPC would give me the impression that the PCs aren't really in danger. A Controller just takes too much spotlight when he is changing the battlefield and popping Minions right and left. And the Striker is dealing the most damage - I wouldn't want to give that job to a DMNPC.
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In my group, we're currently often having only 3 players (plus DM), so we decided to play 4-group parties where one player sometimes has to control two characters. Its an imperfect solution, but it works. In a way, we already started this in 3E, by many PCs picking up Leadership to get a cohort, but it was sometimes a little of a headache to control two characters (especially if you picked two spellcasters)