rating?

The unrated version is obviously not rated but I would give it a decent R.

Violence: Tongues go through heads many times, someone's eye gets put out from behind her head, ummm.... oh, an alien gets double stabbed through the stomach and an alien gets blasted with a shotgun. Another dude is hit with a cab and pinned to a wall before being doused in gasoline and set on fire. Yeah, that's about it.

Sexuality: Two sex scenes, one is about 20 seconds and the other includes flashes of the alien form having sex, which is very distracting and takes all the hotness out of the scene.

Nudity: You see a total of three different sets of boobs but none of them are on the screen for a significant amount of time. A lot of the time, the nipples look more like burn scars, obviously to cover them and protect the future careers of the women in the movie (ha).

Language: a few curses here and there until about an hour in, when the main character starts throwing "*beep*" around a few times. Note that these feel a little out of place and probably could have been left out.

All in all, more tame than the second one in terms of gore and nudity, more tame than the nudity of the first but more action (and nudity) than the third, although that's not saying a whole lot.

In Finland there is a professional organization that rates films called Valtion elokuvatarkastamo (VET). The people working there are media experts and child psychologists and they rate and evaluate every film that are sold or shown in Finland (and they also rate television programs and video games)..

This is how they have reviewed this movie:

K15:
"Tieteisseikkailussa yksityiskohtaista ja veristä väkivaltaa, jonka vaikutusta hirviöhahmojen epärealistisuus lieventää. Lisäksi pari avointa, yksityiskohdiltaan peiteltyä seksikohtausta."

In English that would be (translated by me - no official translation available):
In a Sci-Fi adventure there are some detailed and bloody violence, which effect is moderated by the unrealism of monster characters. Also some clear sex scenes with covered details..

Rating K15 means that it is not suitable for a child younger than 15 years old.. But you can watch it when you are 13 if there is an adult present.

There is also a much higher rating in Finland that is K18. One rating lower from K15 is K13..
So K15 is probably quite equal to the R-rating in USA - just that in Finland some things don't affect the rating so much (eg. nudity) and some things affect more than in the USA (eg. violence and gore).