IntelliJ's autocomplete is much better, but it's doing some heavy lifting to get such good results.
I haven't gone crazy testing ST4 btw, I played with it for like 20 minutes. It felt like it was maybe 65% between ST3's auto-complete and what you'd get with a LSP back-end using VSCode. Meaning it's closer to VSCode than it is than ST3, but not as good as a LSP, but there's also no effort to get it set up so it's a reasonable trade off if you're someone like me who just wants pretty good auto complete without configuring a million tools to run in the background.
