You are not a proper sample size
. I've heard that DMC4 was meant to appeal to mainstream audiences but I don't really see it... Only thing they did was add Nero so that newer players could start fresh with the series.. Hell I didn't get the game until a year after release (prob because I didnt have a ps3 or xbox)..
So looking back at 2008 when DMC4 came out it was competing with Assassin's Creed 1 and Prince of Persia reboot. The Prince of Persia reboot (hmm seems familiar) sold pretty badly, under 1 million units, and AS1 sold 5 million units, so in comparison to a brand new series (AS1) DMC4 had mediocre sales. But I assume DMC4 wasn't advertised as much... Now in 2013 you have AC3 which just came out and MGR competing with DmC. So almost a similar set of circumstances except DmC is now the reboot!