I think TS4 definitely has at least a good 2 years left in it for more content. As already mentioned: We still need a fairies GP to complete the occult collection.
We really should get cars at some point. If they did the tedious work to make horses, then cars shouldn't be an issue either. And yes, the game could really still use a hotels pack, bands, and also some kind of futuristic and/or into the past kind of pack to match the content of what the previous games offered.
I do think that TS5 has been in development for a couple of years too though. The first year or two was probably making all of the concept art and planning the basic core game mechanics. Then of course they have to hire a team for all of the separate departments and have to also create the basis for the game to run on. We already saw an idea of what the worlds could look like at the summit, and it definitely was intriguing. I hope we get more insight into what the worlds may look like and some basic BG ideas in the Behind The Sims video, though I know the event itself isn't long so I'm guessing the details would be a 5 minute segment at max.
Lastly, it is apparent that the team working on Project Rene is separate from TS4 team, meaning both games are being worked on simultaneously. Working on TS5 shouldn't interfere with the TS4 team's work, so I definitely think they could have gotten quite a lot done on TS5 in the last half a year.
