Another reason I probably will never implement a commenting system here is that I am just the sort of person who would obsess over replies, or the lack of them. This is not interesting in itself but what is interesting is that it actually has a name.
Last time I said "full replacements of Doom or Doom 2 would have over 30 maps" which, after it was posted, I realised was only true if you include the Ultimate Doom's fourth episode. Whether or not you do this is open to debate; its plot (if Doom can be said to have a plot) is so unpleasantly grafted on to the original that it is much easier to just think of it as a PWAD. It's not like you can play straight on into it from the end of E3M8.
Let's flesh this out - Doom's plot is that you're stationed on Mars with the UAC, who have discovered "gates" on Mars' two moons and are experimenting with teleporter technology; unfortunately the "space between the gates" is Hell. Deimos vanishes, Phobos is overrun by monsters. You fight your way through to Phobos' gate and enter it, finding yourself on the lost Deimos base. This it turns out has been dragged into Hell, which you must now pass through.
At the end you kill the spider which masterminded the invasion of the moon bases, and are rewarded by being transported back to earth - you are told this at the end of episode 3, after which, you see the famous "bunny scroller" and a teaser for Doom 2. It's clear you're meant to be back on Earth.
Once you play episode 4, whose sky texture is from the bunny scroller, you get more text. It says the spider sent out its legions before you could reach it, but like a true hero, you killed them anyway. Also, it implies that you did this because "someone was gonna pay for what happened to daisy, your pet rabbit."
The problems are that where all of this is meant to have taken place. Supposedly it must be some kind of a small old-fashioned town or something, situated right by where you appear when returned from Hell. Furthermore the part about the rabbit can't be anything other than a daft joke. Come on, pet rabbit?
The point is, every way I try to think about it, I can't make E4's plot fit with the previous three episodes, and Doom 2, and it is much easier to think of it as "just a PWAD", and the original Doom only really has 27 maps.