Does anyone know a good way to keep leeches alive during a 4-week trip, or any other type of live-ish bait for the smallies/eyes that WOULD last that long?

If you change the water daily, keep them in fresh water, I do not see why they would not last. That is a long time to carry around leeches with you. How many pounds are you taking?
Have you ever considered artificial baits?

I'm not sure how many pounds yet, I will probably be fishing an average of every other day. I have used artificial "live" baits but haven't had much luck with them. I can barely get a few casts in with casting lures without snagging a rock in the BWCA, so I have been mostly jigging up there for the last few years. Are there some good putty/fake "live" baits that work for anyone?

I've done two weeks with leeches plenty of times. Usually turned into a hurry up and use them before they turn grey towards the end. I'm pretty anal about keeping the leech locker submerged and not introducing sunscreen/bug dope when handling. Portaging and travel days take a toll IMO.

I like the gulp products, just never had any luck with their leeches and the action is poor on a jig.

I'd personally bring a minnow trap and a bait bucket if it was the BWCA to trap crawfish/minnows. But I'm into high maintenance activities like that to pass my time while at camp relaxing.

You will need to get one of those yellow screw on lid leech holder. When fishing you just remove the amount you will need for the day and put into a mesh holder. Sink the leech holder in deeper water so the turtles and otters don't get to them. I tie a rock onto the holder to sink it. Leeches can live up to one year without food so you don't need to feed them. You will be fine keeping Leeches this long.

quote PurpleHeartPaddle: "I'm not sure how many pounds yet, I will probably be fishing an average of every other day. I have used artificial "live" baits but haven't had much luck with them. I can barely get a few casts in with casting lures without snagging a rock in the BWCA, so I have been mostly jigging up there for the last few years. Are there some good putty/fake "live" baits that work for anyone?"

Call the bait shop in the morning, and we can fix that snagging you speak of.