Good question!
Lately my favorite 4mm travel linear has been the Gateron North Pole - aside from looking like a wet cube of ice, it’s glassy-smooth with a relatively mild bottom-out. It’s more or less a Pro 2.0 Yellow with a thicker polycarb bottom housing, slightly heavier weighting in practice, and a stem made of Ink plastic. I have a full review of that switch here.
Here’s a recording of those in my Portico with some Cherry profile PBT caps:
Another of my favorite full-travel linears is the TTC Ace, which at one point was my favorite switch. These days factory lube has gotten pretty darn competent, but these were the first factory linear I tried that I thought was totally solid out of the box. Not outstanding in any one area per se, but outstanding in that I found no significant caveats - nothing about it sucks, ha. These are pretty normal in a lot of ways, but feel above-average in terms of stability and solidness to me. That said, of those I’m listing here today it does have the most hard bottom-out, being in pretty standard territory there.
Here’s a recording of those in the same Portico with a different set of Cherry profile PBT caps on:
Another worth listing is NovelKeys’ Silk series of switches; JWK linears pre-lubed with 205g0. They’re probably the most standard of the bunch, but quite well-executed at that. If you just want a no-messing-around smooth MX-pattern linear, you could do a whole lot worse than these. Compared with the Aces, I’d say these are a bit more smooth if a bit less stable.
Here’s a recording of them in a Portico68 BL with ABS Cherry profile caps - actually a very different board from the previous two recordings, having a flex-cut polycarb plate and aluminum case:
One more I’ll mention is the Momoka Frog - it’s not as grain-free as the NP or free-sliding as the Ace or Silk, but is still a notably low-grain switch. Sort of like the NP and a few other all-clear switches such as the Aqua King, Frogs have a sort of smooth resistance in the travel - perhaps a very mild gumminess, for lack of better words. Unlike Aqua Kings, however, this aspect is reasonably consistent between switches. Probably the most notable thing about these is now not harsh they are despite having no dampening. That, and the fact that they actuate somewhere past 2mm, closer to bottom-out. Overall I think the NPs are my favorite of these four and might represent the best overall balance of features - but if you want to prioritize being kind to sensitive fingers without crossing-over into the land of mush, these Froggos might be the best bet I know of. ThereminGoat has a full review of that switch here.
Here’s a recording of them in an NK65EE with ABS MT3 high-profile caps: