Thanks to @HeatherWaite8 for this art piece!
At level four, nearly every class eschews getting a class-based improvement and grants the player the ability to choose whether to take a Feat or gain an Ability Score Improvement (ASI). The only two exceptions are druids and monks, who also get a class-based improvement (Wild Shape Improvement and Slow Fall, respectively). Most of the primary spellcasters (bards, clerics, druids, sorcerers, and wizards) also receive an additional cantrip and an extra second level spell slot per day. Warlocks just get the extra cantrip, but they, like bards and sorcerers, add one spell to their known spell list in addition to the cantrip.
If the player chooses to go the ASI route, they can choose to either give themselves a permanent +2 to one of their ability scores (Strength, Dexterity, Constitution, Intelligence, Wisdom, and Charisma) or a permanent +1 to two of their ability scores.
If the player chooses to go the Feat route, they can choose one of the Feats listed in the D&D handbooks, like the Player’s Handbook and Xanathar’s Guide to Everything. Sometimes these feats give the character specialized abilities, sometimes it allows the player to increase one or more abilities or skills (like athletics or passive perception), and sometimes they do both. Some Feats can be achieved by anyone, while some have race, spellcasting, or ability score prerequisites. For example, Prodigy (which Beau took as part of her variant human traits) can only be taken by humans, half-orcs, and half-elves.
Beau and Yasha
At level four, both Beau and Yasha took the same feat, Sentinel. This feat is designed specifically to aid melee attackers. If a creature provokes an attack of opportunity from them, and they hit, the creature’s speed becomes 0 for the rest of that turn. To the annoyance of other monks and rogues everywhere, Beau and Yasha will still be able to make attacks of opportunity on opponents, even if those opponents take the disengage action. Finally, when a creature within five feet of them makes an attack on someone other than them, Beau and Yasha can use their reaction to make a melee attack against that creature (this is not considered an attack of opportunity, since the creature didn’t move away from them). We saw Beau use this ability in Episode 2-15 against a Will-o’-Wisp that attacked Fjord and the Gelatinous Cube as it attacked Yasha.
Thanks to @Bill_Wash for this art piece!
Caleb opted to increase his intelligence score from 18 to 20, which is the typical maximum to which ability scores can go. He’ll now have a +5 modifier to his intelligence checks, and will increase any intelligence-based skill checks by +1. Additionally, his spell save DC and spell attack modifier will also increase by 1.
Thanks to @planarbindings for this art piece!
Fjord, as Travis mentioned on Talks Machina, took the War Caster feat. In Campaign 1, both Scanlan and Keyleth were War Casters. This feat has a prerequisite that the character can already cast at least one spell. Fjord will now have advantage on concentration checks for maintaining his concentration spells, like Hex. He can now perform the somatic parts of spells even when he’s got weapons in his hands, and he can now cast certain spells as an attack of opportunity. Spells must take one action to cast and only damage the creature who provoked the attack. Eldritch Blast would be the only spell Fjord knows right now that would fit into this category.
Thanks to @Zephyri for this art piece!
Jester, like Caleb, took an ASI. She opted to spread her increase between her wisdom and dexterity scores, which are now both 18. This will give her a +1 increase to any wisdom or dexterity-based skill checks, like perception, insight, acrobatics, or initiative. It will also increase her spell attack bonus, spell save DC, and AC by 1.
Thanks to @leanart_ for this art piece!
Molly, who damages himself in nearly every battle in order to utilize his blood hunter abilities, took the Tough feat. In Campaign 1, both Scanlan and Grog took Tough. This feat increases the character’s hit point maximum by twice the player’s level. This means that, at level 4, Molly gained an additional 8 hit points to what he rolled on top of his Constitution modifier. Every level increase after this will also grant an additional 2 hit points to whatever he rolls.
Thanks to @bailierosenlund for this art piece!
Nott wields a hand crossbow, which can be used with one hand, a unique property among crossbows. She gains the most benefits possible, then, from the Crossbow Expert feat that she took. She now ignores the loading property of the crossbow, which typically requires a bonus action, and she no longer has disadvantage from firing in melee range of another creature. Finally, she can now attack with her hand crossbow as a bonus action when she attacks with a one-handed weapon first...like her hand crossbow. This effectively gives her an extra attack. She got to use every single one of these new benefits during The Mighty Nein’s battles in episode 2-15.