Echoing the above sentiments: Zilents have a sharper tactile event (P-shaped), while Boba U4s have a rounder tactile event (D-shaped). To me, both switches are excellent silent tactiles, with the Boba U4s being that bit quieter than Zilents. If you’ve used Zealios and really like that tactility, then Zilents are the natural silent alternative to them. However if you prefer something a little more rounded in tactility, Boba U4s should be the switch to consider.
If you’re just looking for the quietest tactile switch, the most silent tactile I’ve used thus far (and I now daily drive) are lubed Aliaz 60g in Cherry black housings. They are orders of magnitude softer than Boba U4s, to the point that they can be “unsatisfying”, since Zilents and U4s have a nice “thud” whilst you don’t hear anything with Aliaz. The tactile event of Aliaz is more similar to MX Browns, and thus are way less pronounced than Zilents and U4s.