I can't speak for the 4Ai but my 4A custom doesn't sound anything like the W4 I used to own. I've never heard the ER4 but from what I've read Ety earphones lean more on the analytical side which Heir gear does not. Oh well, maybe I just need to hear an ER4 to understand where you're coming from.
Unfortunately, all IEMs' are fit dependent. And as my ear canals seem to change day to day I sometimes wonder if I hear things the same every day. That said, my hearing is usually in line with Jokers. His descriptions are pretty much how I hear a lot of these headphones with the exception of a lot of the larger bore stuff like Hifiman and UE. I can't really get those to stay in my ears. The Heirs are borderline as far as staying put. Not an active headphone by any means. I didn't have the W4 and 4.ai at the same time ( and I expect there are fit/sound issues between the custom and universal version of 4.a/4.ai) , the W4 is warmer. From what I recall the 4.ai was leaner, more "neutral". I modded my W4 with the extra filters, it did change the sound somewhat.
So I base my standards on Etymotic as a "neutral" 'phone. They have a spike at 5-6000 Hz I'm sensitive to. The Hf5 reminded me of the Hifiman Re-zero. A little warmer, still basically "neutral". The ER4 is a little more analytical and "cold".
The 4.ai had more bass I believe than the ER4, (therefore not neutral?), the transparency and imaging were very enjoyable. I liked the headphone a lot even though it really doesn't go well with crust punk and black metal (ouch).
I wouldn't go as far as to say "nothing like " the W4, but the W4 is more mid-happy, warm upper bass typical Westone/Shure/Earsonics than a cold/neutral Etymotic. They are different. The 4.ai in my minds eye was more analytical, sharper, perhaps cleaner. I like both for different reasons, but the W4 is hardly the "bass light" treble happy IEM I was led to believe. Quite the opposite actually. In your terminology I'd call it a "fun" IEM.
My point with the earlier post was to point out the 4.ai is less like the SM64 than a W4 or the like. My experience with the 4.ai made it a sharp, clear IEM with treble emphasis. SM64 probably more sub -bass emphasized. The quads in my experience all sound to me like they're going for a similar experience of a detailed/neutral presentation. In my order of preference it was 4.ai>W4>UE900. Again, my W4 is modified, but I get better treble extension as a result. The UE900 was slow and muddy for my music, I was really surprised at how much better the 4.ai was in speed and clarity. It could have been fatigue, but I really didn't like them.
The W4 sound more "neutral" than the SM64 but still on the warm side. The 4.ai was closer to neutral in my memory. Hence the comparison suggestion.
To me it's all about how sensitive you are to subtle differences, I listen to a lot of different music, the IEM has to cover a spectrum. I can't specialize in "these are my classical, these are my rock, these are my jazz" type of setups. That leads to compromises.
So to me yes, the W4 is similar to the 4.ai. The SM3 is similar to the SM64 which is similar to a UM3x.....
The little differences on frequency emphasis and dips denote the changes. The 4.ai is closer to an ER4 than the SM64. THe W4 is closer to the Um3x than the ER4. Is the W4 then close to the SM64 in sound ? No I don't think so, but I guess it depends on threshold. At least I can compare the W4/SM64/ER4 with an equal footing due to the bore and therefore fit.