Well, the whole concept of "evil" is subjective. However, I think the following would qualify as fitting standard "Western archetypes" of "evil:"
1. Getting to the end of a quest for a follower that they clearly want completed, getting everything out of if that you want, and dismissing them before they get their closure.
2. Encountering a group of mourners at a funeral, pickpockting and/or shooting them all in order to loot their corpses.
3. Killing anyone you can and cannibalizing their corpse!
4. Telling Preston and the Quincy Refugees to jump in a lake and leaving them sitting in the Concord Museum for all time (with console you could even kill them if you like).
5. Getting aboard the Prydwen, receiving the knight rank and then assassinating Maxson and killing every single BoS onboard and at the base below.
6. Ditto for any of the other factions: gain entrance/trust, then kill them all.
7. Robbing/Pickpocketing anyone and everyone who has property.
8. Telling Dogmeat "You are a smelly mutt" and kicking him to the curb.
etc., etc. We could literally list scores if not hundreds of individual acts of "reprehensible behavior" which the game is perfectly willing to allow you to engage in.
So the claim that in Fallout 4 you "CANNOT be evil" is absurd on the same level as Monty Python.
Now, it is true that there are certain archetypically "evil" behaviors one is not able to engage in ingame and moreover that there are lots of "nudges" and "encouragements" if not cajoling and nagging to "do good."
If anything, the removal of "Karma" and the stupid "red-handed" marker for stolen items has done more to facilitate "evil" play than either FONV or FO3.