QUAD 4 simply means a 4 valve per cylinder 4 cylinder engine Oldsmobile produced, and GM put in various cars from 88~94.
A few facts: the QUAD4 was produced from 88~94. While a lot will still refer to the engine that replaced it as a QUAD4, it is not. It became the "Twin Cam". While I can't get into GM's Engineer's heads on the why, the fact you can not bolt a 95 right in where anything from 88~94 was is probably the main reason. Having said that, the basic engine was very much the same from 88~2002, which is why most people will refer to the any 2.3L or 2.4L in those years as a QUAD4. Why does it matter? Well the fact is when Olds (all the engines are Oldsmobile motors byt the way, regardless of the make its in) first made the motor, which was prior to the production model in 1988, they were making 750HP with a 2.2L version of it. (An Andretti drove the indy car it powered to more than 226mph laps at Indianapolis.)
So, for most of us what do we care? We care that we can get v8 power from a 4 cylinder with 4cyl economy when we can keep our foot out of it.
So, why does it get bashed all the time. Well like anything else there were some growing pains which did include some engine failures. More or less those problems have been resolved. Even in the older configurations the most significant issues have readily available fixes.
So, what should I look for? In a perfect world you would want a post 94 block mated to a pre 94 HO head. You would get the best power with the newer fixes and a smoother running motor. In reality, if you want reliable economy, look for a 96 or later twin cam version. (some people will refer to the single cam as a QUAD4. well clearly it is not as it only has 2 valves/cyl) If you want serious out of the box OEM power grab an early 90's HO. It has to ba a VIN A motor (8th digit), and ALL of them will say HO right on the engine cover. It doesn't guarantee anything 100%, but it will have that cover if it hasn't been changed.
So, what should I stay away from? Avoid the early single cam motors like the plague. These engines are where most of the bashing comes from, even thought they aren't a QUAD4. Also avoid sticking with the original head of ANY pre 95 QUAD4. This may sound like a stupid thing to say (ya want me to buy a car and change the head?) but if you're thinking about buying a cheap car that happens to be in good shape, why not spend $300 and take 90% of your worries out in an afternoon. A person with SOME mechanical ability, the right tools (nothing fancy or expensive) and an off the shelf guide from a parts store can do this job in about 4~6 hours depending on the person and work area.
The Quad 4 was once hailed as the powerplant of the future, having the power of a V-8 while the effeciancy of a 4 cylinder. It turned out to be a problematic engine. Before GM worked out the issues with head-gaskets, the ignition system, and other misc. problems the engine had already earned a bad reputation. This is especially because of the single-overhead-cam version, which used a horrible amalgamation of parts and was the worst version of this engine ever made.
The shining redeption of the Quad 4 was its potential as a FWD performance powerplant. It helped Oldsmobile win many SCCA SOLO2 and ISMA competitions, and set a land speed record in the early 90's with the Oldsmobile Areotech. Many hot rodders and tuners thus choose to use the quad 4 because of its potential.