Armor-skinned deathclaws, murder-minded mirelurks, alpha albino relentless: these are a few reasons players should just stay safe in their vaults and starve to a relatively gentle death. However, for those who choose to wander around Fallout 4's wasteland, there are a lot of things to look out for outside.
If it bleeds, you can kill it. Those words hold true in Fallout 4, but not everything in the game bleeds. So here's a better one: know thine enemy.
Feral Ghouls - Like a lot of the normalized oddities in Fallout 4, feral ghouls are relative weaklings to attack in packs. Hit them with pure old kinetic energy for some blunt force trauma, but avoid shooting them up with radiation-based weaponry because they live for the stuff.
Raiders - Like all humans, raiders have their brains in their heads. Shoot for that thing.
Super Mutants - They're still configured like humans, despite the performance-enhancing gamma rays they've been doping up on. Shoot them in the head.
Synths - Don't aim for just their heads. Shoot at whatever's exposed, dismembering and destroying them. Energy weapons work exceptionally well on synths.
Bloodbugs - All they know is anger, but they have to get in close to do something about it. VATS are essential for firing at their thin, mosquito-like frames.
Brahmin - Brahmin will only attack when provoked, so leave them be. Players who do shoot down Brahmin face the risk of angering an entire town and getting killed as a result. Redditor blippetyblop did so, "and the entire town turned on me in an instant."
Cats - Give them a gentle scratch behind an ear, and everything will be all right.
Deathclaw - This is probably what the Montauk monster would have looked like as an adult. The things are bipedal tanks and should be shot at as such. Use explosive and armor piercing rounds. Oh, and bring your armor.
Mongrels - these mangy, feral mutts can be taken down with small arms. Just don't be surprised if what looks like one mongrel turns out to be a pack of mongrels.
Mirelurks - Something like these might really exist in the deepest parts of the ocean. These things are kitted out with some formidable armor, with their backs strong enough to deflect bullets. Hit them in their guts when they open up for a charge.
Mole Rats - With mole rats, it's really a game of whack-a-mole. Hit them with whatever you want. They're easy to kill and they don't hit too hard.
Radroaches - Since they're too big to be stomped on, this low-level threat can be dispatched with melee attacks.
Radscorpions - As with mole rats, radscorpions will get you playing another game of whack-a-mole, but these creatures are equipped with poisonous defenses. Keep this game at mid-range to avoid being poisoned by these borrowing beasts.
Yao Guais - These mutated bears have thick hides and a lot of attitude. Don't go toe to toe with them, as their melee attacks are not to be trifled with. If you really feel like dancing, at least use a shotgun.