Finding myself surprised to like Fourte more than U18 so far. Based on impressions and my own past preferences I would have stayed away from it, as I thought it would be too bright.
I find U18's mid bass to be too prominent in comparison to the rest of the frequencies, and vocals too recessed (both modules). Fourte is decidedly brighter but has deeper and more natural sub bass, and vocals and mids are more forward. Imaging is clearer as are details and a taller, all-around more holographic soundstage. Despite it being brighter and a bit colder it is certainly not dry or boring at all.
U18 bass quality is good until you compare with the Fourte's DD again - I'm sticking with dynamics or at least hybrids for sure. Despite my limited experience with BA bass I find they sound a little like tiny micro-fart expulsion cannons
Despite both IEMs being great in their own rights, neither one will be my next purchase. And, not at all trying to debate whether their price is justified or not, IMO and compared with the only other flagship IEM I've owned, the Vega, they absolutely do not warrant the price difference based on performance. More like a different tuning with perhaps a slight, and I mean maybe like 2%, better technical performance.