Personally, I've always considered episode 4 as a bolt-on extra that doesn't really fit with the original game with regards to appearance, gameplay or even "story". I regard the original three episodes as being what Doom is and episode 4 more like a mission pack like "The Scourge of Armagon" is for Quake (but free, obviously) or, in modern speak, a DLC-like add on.
Finding myself in complete agreement... And yet, this resulted in E4 captivating my teenage brain more than anything else in the game. It was so out of place and abstract I'd immerse myself in those strange, intriguing environments. Whereas the first three episodes leave little to the imagination, the fourth immediately gave me a strong "we're not in Kansas anymore" vibe. No more story, no map progression nor map, not even a common theme from level to level. Nothing reminding you of human presence or existence at all, just a bunch of monsters and you, alone, against them. It felt like wandering off the rails and stepping in an alien dimension.
It took until Sunder to recapture that feeling for me, and no commercial games have done the trick. People want nice, relatable, understandable, human-readable environments. So-called abstract settings always have to be explained, broken down into words, reduced to tiny pieces, with hardly any room for interpretation beyond details, and even then, human-centric ones at that. Disappointing.