It really depends on how smart your players are. In the veteran campaigns I've run before my general rule of thumb is 'Players will hit the enemy 40-25% of the time' and if the boss crits it should put you into bleed out or at least fairly close. That ends up meaning of course that with smart players using flanking, buffs and debuffs usually get about a 50% chance to hit on any given attack and when the boss hits them it feels like it actually matters.
Edit:That said, at level 3 I know people who are rather nooby at table top tactics or have really underpowered characters through either bad feat choice or for story reasons that would have a ton of trouble even dealing with a CR 4 monster. (That said, your kineticist having 40hp sounds like he's more than optimized enough to be able to take on something that much stronger.)
Editx2 Combo:I also just remembered the reason I started to post in the first place. I'm planning on running a sort of semi-play by post game of a system called Blade of the Iron throne. What would happen is that role-play, day to day life, shopping, planning and the like will take place in the forum thread and then when someone enters into combat I would ask them to join me in a discord or roll20 to do the combat encounter. I feel that blade of the iron throne should work well using this because combat takes place in a 'limelight' mode where each player takes turns doing a number of 'exchanges' with whatever or whoever is fighting them and the limelight swaps when something major in the fight happens like the player taking a large blow, killing their enemy, ect. So ideally I'd be able to do a 10-20 minute session with each person individually, post the results in the thread and get ahold of another person who hasn't taken their turn yet to do the same thing.
Blade of the Iron throne itself has an indepth combat system utilizing stances, attack types, and specialty defenses to out-manouver your oppoent. Including causing them to bleed to death from a number of small wounds all over their body (it has 14 attack zones that you can choose from, and then 3 subzones in that area you roll for when you hit) as well as pain, shock and regular old coup-de-grace style attacks. Think dwarf fortress combat the tabletop more or less, including having armor only protect you where armor actually is. Wearing a armored chest-plate ain't gonna save your shins from that warhammer. All that said, because you do not have a true health bar like most rpg's (you just get slower, drained ect as you take blows. Allowing them to eventually get in powerful attacks that kill you outright (even then you have a limited number of drama points to spend to 'not really die just look like you got brutally killed but you can still be saved'))
It also has magic based off of necromancy, daemonlogy, divine powers and supernatural afflictions/curses. The use of such magic requires you to carefully contain your spells, not only cast them to avoid damaging people you don't want to damage and counter Taint which will slowly twist the bodies and minds of careless wizards. That said the spell system has rules and guidelines to allow gm's to make their own lesser mysteries (think low level dnd spells), greater mysteries (level 4-7 spells) and arcane secrets (level 8, 9 and epic spells) so with some work with me you could have any reasonable spellcaster you'd want for the setting.
TLDR:It's a sword and sorcery game set in a brutal world, with disposable mooks, powerful arch-enemies and gigantic monsters that is reminiscent of old literature like the Odyssey, Beowulf and newer fiction like Conan the barbarian.
Overall I have a plan for an alt-history like dark ages Europe setting which I am more than happy to change to accommodate unique character ideas for anyone who'd want to play. I have a real-life friend or two who already have expressed interest in playing (one has a character already made) but I'd be looking for 3-4 more players from the forums if there's any interest?
Does anyone have any questions? Feedback? Here's a link to the extract first chapter of the rulebook as put out by the publisher
here and I have the character sheets and full core book on pdf if you have questions.