grand theft auto 4...Is a success or a failure???

[Ail]

In the end you can't expect every well known gaming franchise to keep selling more with every new iteration.
As the industry matures the competition is becoming fiercer and it is hard to keep repeating past success.
The fact that every 5 years or so a new generation starts and the user base isn't the same in size for every new release makes comparison difficult too.

However GTA4 has demonstrated that the franchise is still very powerfull, in very good health and very profitable...
As for development costs I persist, I believe GTA is the kind of franchise where the developer has to invest more every sequel as it is what the customer expects. When you play GTA4 you want to be wowed and those tiny little details that keeps achieving that effect ( like those shows you can watch on the TV in game) is what in the end differentiates GTA4 from any possible competitor...