I owned a 1984 rear-drive Cadillac DeVille six-door limo with a HT4100 engine, owned it for several years as my daily driver, and would still have it if not for the car having gotten hit. In other words, my not having the car any more had nothing to do with the engine.
The car had something like 120,000 miles on it when it got hit, and the engine was still going fine. I was the third owner on the car, with the two previous owners being funeral homes, and when I got it, the glove box was full of receipts of work done on the car, which I went through, and it had always been a maintained car.
The 4100 was a real dog in the acceleration department, and on the expressway, the car topped out at 75, but the engine did perform wonderfully at its intended purpose of being good with gas mileage.
The horror stories regarding the HT4100 were not so much a result of any flawed design, but rather the result of most owners not following a slightly different maintainence schedule for the car. Most people just turn the key, and as long as the car starts, it gets no further thought. Well the HT4100 required its upkeep, and when it didn't get it, problems were had, some of them serious, and naturally it had to GM's fault for building such a lousy motor, and had nothing to do with owners ignoring what the owners manual said to do.
As I recall my HT4100 owners manual, it called for oil changes every 2500 miles instead of the more common 3000 miles, and whenever the coolant was changed, you HAD to add the GM sealant tablets to the cooling system. Other than those two quirks, and the fact that flooring it from traffic lights not only put undue strain on the engine, but also didn't accomplish anything anyway, I really have nothing bad to say about it.
The Cadillac V8-6-4 is also an engine I have experience with, and is also an engine that most people, even Cadillac experts, are mistaken about. The V8-6-4 was NOT a unique engine. What it was, was the Cadillac 368 V-8 fitted with a unique fuel injection system that would cut off fuel delivery to two cylinders, and then two more cylinders, when certain speeds were reached. Problems reported included cylinders deactivating too quickly resulting in a car with poor acceleration, a momentary but noticable hesitation when cylinders went offline, and some even reported fuel delvery being cut off entirely on occasion. The 8-6-4 fuel injection was governed by the car's speed, and simply unplugging the sensor from the transmission resulted in the engine acting as a full-time V-8 motor.
The 8-6-4 was a one year only system (1981), but the 368 motor it was fitted to was available in carburated form from 1980-1984. Additionally, only passenger cars got the 8-6-4 system in 1981, the commercial chassis in 1981 remained a carburated version. Any 1981 hearses that have the digital MPG readout display on the dash are ones that were converted from from sedans, not commercial chassis cars.