Was just listening to the Bonus EXP podcast, and they were talking about Civ 5 a bit. Andy basically said he already had 4, so why bother? 
It got me thinking about franchise reboots or retools, and how many games a series can go before it's time for a reboot. Based on the few games I was thinking about, it seems like 4 is a good number. 
A few games I was thinking of: 
Devil May Cry went to 4, and is now coming back as just Devil May Cry again. 
Civ 4 to Civ 5 
Whatever the next Halo game is (you could almost count odst as a reboot, except they really didn't go anywhere with it, and essentially finished a story arc with Reach) 
Call of Duty 3 to 4 (I'm discounting the 1st CoD, and the portable releases in this case) 
Ratchet and Clank: went four games, and rebooted as Ratchet and Clank future, which is on tap to get a 4th game in the series. 
Splinter Cell 
 
Obviously there's franchises I'm not familiar with that may add or subtract to this idea, but I'm curious to know your thoughts on if if there's a reason 4 seems to pop up the way it does, and if there are games that maybe you wish would have had a reboot that didn't get one.