4k is just the digital master
Everyone with an ounce of sense (so, that's everyone other than George Lucas, who doesn't want his Star Wars trilogy to look embarrassingly better than the prequels he shot digitally at 1920x1080) is scanning and mastering at 4k these days where possible. The difference is visible on big cinema screens, and a nice bit of anti-aliased scaling down to Blu-ray resolution looks nicer than from a 2k source too.
The biggest quality problem being faced is that even if you're not Lucas testing out shoddy new camera tech, a lot of late 90s to mid 00s films went through a 2k digital intermediate, and so the source data just isn't there. That's why the current Blu-ray of The Fellowship Of The Ring looks all waxy; a straight comparison between the Bridge of Khazad Dum shots in it and the beginning of Two Towers shows a huge difference as the tech had come along in that time.
As for Titanic, it's quite fun in a spectacularly dumb way once the ship starts sinking, so I might grab a 2D Blu-ray (somehow I've manage to lose VHS, DVD and even Laserdisc copies to other people over the years). But I hate 3D films, so I'll skip the cinema run.