1/2NL, about 250 effective stacks with main villain. SB appears about 50 yo. 3 limpers and I limp BTN with KJo. Flop comes KJ4,r. Checks to me, I bet 10, SB calls and others fold. Turn is a 6 completing the rainbow, SB checks, I bet 20, SB raises to 40, I call. River is an 8, SB bets 40 and I call. Should I have raises preflop or raised the turn or river?

yes

This.

im not sure how youre not all in by the river. you really only lose to one hand (44). id put villain on K8 or J8, or maybe even a weirdly played AA.

pre i could go either-or. in a high rake game probably raising pre is better to make the pot bigger relative to rake %. obv if someone limp reraises just fold.

Preflop is a raise for sure, you'd have to describe very particular circumstances where it's not. So when you flopped top two pair you'd make a plan to start piling it in.

Min-raises can be suck bets with a set (44/66) or "see where I'm at" with a strong one pair (AK/KQ) or two pair. As such, I'd prefer to call the turn and let villain bet again on the river
The river bet size looks more like a nervous one or two pair so you could probably click it to $100 and get a crying call.

Yeah, I thought afterwards I should have raised the small bet on the river. He should raise KK/JJ preflop and I strongly block those, so just losing to 44. He showed K4 and acted like he expected it to be good.

sorry misread your post, i meant K6 or J6

I don’t hate the line again at some V’s and of course increasing the action on the turn and/or makes sense against a lot of V’s.

I like this hand.

Preflop doesn't hugely matter. I would mostly raise over three limps with KJ. I'd also raise, say, KTs and QJs, but overlimp KTo, QJo, all QT and JT. But those are ballpark and overlimping a modest offsuit hand can't be bad.

In game, sadly I would probably default to a "2 pair isn't strong enough to 3bet" mentality but this is almost certainly wrong. You lose to almost nothing. SB should never overlimp KK or JJ, and shouldn't ever call the flop with 66 with three players still to act...so apart from 44 you have the nuts. The main hands you're up against here are 44 (which you lose to), QT and K6 (both of which you beat, and which will call a 3bet). So yeah you can come back over the top on the turn. There might be some other hands out there but they probably aren't betting the river again for you.

River raise now feels a bit thin as QT won't call although the timid river bet size doesn't feel like QT. It's probably very close. The runout has been very safe and top two is still very strong on this board.

Mandatory raise from the BTN. KJo is not a hand to play a 6-way limped pot.