Considering the things that 13-year-olds today see, hear, and experience on a daily basis, TS4 is quite tame for a Teen rated game. The ESRB rates games as being suited for teens with the following criteria:
Content is generally suitable for ages 13 and up. May contain violence, suggestive themes, crude humor, minimal blood, simulated gambling and/or infrequent use of strong language.
It is indeed possible for a game to be rated Teen and still have elements of blood, nudity, and alcohol. Just
check the ESRB website and search for games rated Teen within the past month.
As for whether I think the next game should be rated Mature, I welcome it. However, I doubt it would actually happen.
