The thing with the 4.0 I6 is that it's a very reliable (albeit inefficient) engine, and I've found makes sufficient power/torque in its applications. Sure an LS would make more, but the 4.0 is adequate.
In the case of the Discovery, the engine is the trifecta of bad. It's unreliable, makes insufficient power, and gets bad mileage. Not that I'm worried about mileage, but good mileage would at least make the other two areas feel a bit more tolerable.
Now, I do like the 4.0 as an engine because it's a compact, lightweight V8 that when uncorked makes a good sound, but it belongs in a less stressed application, or else put in an application where reliability is an acceptable sacrifice for weight and the power output is adequate for what it's in. A lot of people like to use these engines for V8 swaps into RX-7s, Miatas, MGs, etc. - it's a perfect fit for those (no I don't want to throw this engine in the RX-7). But a 5,000+ lb SUV? No, it's just not the right engine.