It's all about personal perception.
Even with the added bass, I can't consider the W3 funner than the W4, due to their mildly artificial presentation alone, not to mention the somewhat recessed highs.
You say the W4 are less analytical because they're marketed towards casual listeners.
Then how are the UM3X more "fun" in sound signature, if they're being marketed for "reference monitoring"?
Both are excellent in clarity, separation, detail, what varies the most is the bass (lower on the W4, despite the added bass driver - incredibly detailed, though) and the presentation.
Fun, ear-catching monitors, for casual listening, are the Westone 3. Those are very, very different from the W4. The W4 were not the next in the same line.
The W3 vs UM3X, casual vs reference monitors comparison, makes perfect sense.
In the case of the W4, we're not in the presence of a fun, v-shaped, casual listening, ear-catching monitor. That's why I wouldn't know what to expect form an hypothetical UM4X