The aliens... That's a toughie on scale because they varied in the films even, but the costume was actually a guy in it in the original alien, and also in Aliens too I believe (at least at times), and so the height's about that of an average dude...
I've never compared the kenner figures to Hasbro SW figures, but if I held one of those aliens next to VOTC Han, Bespin Capture Han, etc. and it matched up fairly well (little taller but looked like an average size guy in a big costume), then I would think highly of the scale...
I want to get some of those figures. Some are in stock I just am not real high on fundage right now... I could definitely at least use an alien and a predator... Just something for a little taste of what the others would be like.
This talk of 4"-ish scaled figures really has me jonesing even more for figures in this scale.
LOTR right now would be my ultimate pick for the 1:18-ish scale treatment... Star Trek would be right up there though so those sound intriguing.
What I like about those kits you showed me Mr.PL is that they appear to actually yield a VERY poseable figure. As poseable or more so than most any SW figure short of perhaps a SA figure. I'm intrigued and may just get a couple of what they do have to test them out... Pose 'em next to a Stormie squad like they're ripping them apart.