Though the base models were still underpowered, with a V6 engine making 145 horsepower for 1994 and 1995 and 150 horsepower for 1996 thru 1998, the GT models were much better. They got a V8 engine making 215 hp. The Mustang Cobra was the top model for the line, with a 5.0L GT-40-inspired engine making 240 hp. Motor Trend liked the redesign so much that they named the 1994 model Car of the Year.
As the years went by, the engines just got more and more powerful. Remember how low gas prices were in the mid-to-late 90's? The Cobra ended up with a DOHC V8 engine making 305 horsepower. It was now truly a performance car that could compete with the likes of a Corvette.
The "new edge" look came to the Mustang in 1999, giving the car faceted lines to replace the smooth designs of the earlier cars from this generation. One popular special edition from this era was the Mustang Bullitt, a car inspired by the one used in the old Steve McQueen movie. The Mach 1 was also made in 2003 and 2004.
Some of the earlier 4th generation models now sell for very cheap on eBay. We're talking a couple grand or lower. The later models are a little more expensive, although you can still find some great deals. The special editions are hard to find and demand considerably higher prices, but I think the Mach 1 and especially the Bullitt stand a good chance of being very collectible some day. And don't forget about the Cobra R.