Many people including me at first thought that Luigi was terrible in Smash4. He just felt really bad while playing him. His downB got slowed down, it was more difficult to mash and you couldn't do it at all without a second jump. His upB killed at 80 earliest, etc.
But that was just wrong. He was probably still top 15-20 at the time, it's just that pretty much nobody really tried to play him or got used to his new style.
I started playing him at the end of the 3DS days (shortly before the first patch and the Wii U version) and I can tell you this:
- he still had his nice dthrow combos (though not as consistently, but most of the time almost worked the same)
- he still had his great Fireball and dashgrab, and jabs, and usmash, and like almost everything he has now...
- though he did have worse KO power because of vertical vectoring. Dthrow to downB was probably not even a combo (at KO-%) and couldn't really KO, and usmash and upB were kinda weak, same with fsmash (like most KO moves that KOed off the top).
His fair got a bigger hitbox which was awesome, but his fair was still a great move before regardless. His biggest gains were no vertical vectoring for mostly vertical-hitting moves. DownB being able to KO, dthrow having easier followups, upB actually being useful, etc.
But even without all of this Luigi was still a good character. I'm sure you can imagine this with how good he is now (even though he might be a little overrated imo). He still had dthrow to double fair or fair -> uair etc. Luigi isn't only good because of being able to KO you with dthrow to downB (which is only somewhat reliable in the first place against many characters). And yeah his weak attacks were giving him some trouble which made him worse, but that still doesn't make him that much worse.
Keep in mind every character had a harder time KOing because of vertical vectoring and the blastzones on the 3DS are bigger (further away) too.
Here's a friendly match I had in the 3DS days if you want to get a quick view of what Luigi could always do:
https://www.youtube.com/watch?v=BjPSKXtlowc [not the best quality though] (this was when I first started picking up Luigi and only had a little plan of what I'm doing (like ~2-3 hours of playtime))