I play a huge rotation and many sims I have known since they were babies and their characters have developed slowly (we're talking months here, possibly a year). I spend their childhood and teenage years trying to figure out what they might want to do in life so I usually have an idea when they become young adults, but it's just a rough sketch. I like to take hints from the game and see what happens. For new sims created I normally have some sort of background story in mind, but might not really have an idea what they're going to do and they may fall to be just background characters for my main stories.
In general I don't want to plan too much ahead. It's nice to have an idea of what's going to happen, but I don't like playing though a story with prewritten twists and turns. I wish TS4 offered more random/unexpected stuff so I would have more things happening without me planning them, things that could change the lives of my sims. The recent packs have shown some improvement to this direction, for example the phone call where you're asked to get married in a week. This is exactly what I'm looking for in a sims game. There have been similar additions at least since Get Famous which offered the job pop ups with much harsher end results (actually getting your sim fired). Both of these examples are optional and I would still like there to be something more dramatic, but they'll do.