The bare minimum is to start on Cold Steel 1 if you're planning to play CS3 and CS4.
The endless debate of "Where can I start?" for the Trails series is a consequence of the ambitious method of storytelling that this series is doing (One grand story told across multiple games over the past 15 years) and its inconsistent availability on platforms (PC will be the only way to play every entry on the same machine).
The selling point for this series is how interwoven every entry is to each other. It doesn't cop out with the "This game takes place on another planet" or "It's been 1000 years since the events of the past story arc". All these games have happened within a span of a few years story wise and take place in countries/kingdoms that are next door neighbors. The stories, characters, lore, and world building are connected to each other from game to game and story arc to story arc.
As a result, every reference, callback, introduction of characters (yes even the NPCs sometimes) from previous games, and even slight cameo appearances to previous entries are a whole lot more meaningful and can bring more major story implications compared to other game series.
Thinking back to CS3 thanks to this thread, I remembered what happened at the end of the first chapter in CS3. I don't think that it would have had the same impact to me if I didn't know the significance of the characters that appeared or the setting that it all took place in from playing the previous Trails games.
Right now I'm imagining one of my friends who's planning to get into Trails if he decided to jump into CS3 and saw the end of the first chapter. My reaction was "HOLLLLEEE SHITTTTTT" and his probably would have been "...Who the fuck are all these weird anime-ass looking dudes?" LOL
Someone mentioned Lady Virgilia's youtube channel. I watched her story summaries of Zero and Azure before starting CS3 because I rather wait for either official translations or the fan translation to be done at the time. She does a great job. Even so, she warns you that she skips story stuff from sidequests so understand you ain't getting the full story but close to it. I'm really happy that I did because I'm sure a lot of the things I experienced in CS3 would not have been as impactful without knowing what exactly happened in the Crossbell story arc.
So to answer your question OP, see if you can at bare minimum start out with CS1 and 2. If you can't but you have time to watch some videos, Lady Virgilia's yt channel has story summaries that you can watch. I haven't watched her videos of the Sky trilogy but based on her work for Zero and Azure, they should be fine.
Personally, I don't think the Sky trilogy aged well gameplay wise but I played them before speedhacks were added into them so I bet that helps alot.