The first generation 4Runner was sold from 1984 until 1989. As I mentioned before, it was a pickup truck with a fused shell over the bed and a row of seats in the back. They were only sold with 2 doors, and they could be classified as a compact truck. The standard engine was an I4, with a V6 optional. A turbodiesel I4 was sold in some select markets. Mechanically, these 4Runners were the same as the Toyota pickup being sold at the time, with no changes under the hood.
The second generation 4Runner was sold from 1990 until 1995. This was a major redesign and reconfiguration for the vehicle, as it finally became a real SUV with 4 doors on nearly all models sold in the United States, although a rarer 2 door model was sold in 1990, 1991, and 1992. The standard engine remained a 4 cylinder, but once again a V6 was optional and a turbodiesel sold in some markets. 4 speed automatic and 5 speed manual were still the only transmission options.
The third generation was sold from 1996 until 2002. This 4Runner was more of a luxury SUV than its predecessor. The styling remained similar, but the options and interior received a major upgrade. These models were only sold with 4 doors. The engine options remained the same as before, albeit with more power from each engine choice. These SUVs shared a chassis with the Toyota Land Cruiser Prado.
The fourth generation 4Runner has been sold since 2003. This is the first new generation that is targeted at the same audience as the previous. The major changes were in styling and in the chassis. Some of the trims on the latest 4Runner models include SR4, Special Edition, and the 4Runner Limited. The I4 engine is gone, replaced by a standard V6 engine. A V8 engine is available as an option for the first time.
As far as safety is concerned, the newer 4Runners are very safe, while the older truck-based 4Runners are very unsafe. A third generation model or newer would be recommended if you plan on driving a family around in it, while earlier models might still be fine for use as a utility truck.
If you are looking to buy a used Toyota 4Runner, I would recommend checking out eBay listings before going anywhere else. There are also lots of parts listings on the online auction site.