So I'm in the process of making a campaign, and one of the plot points is some kind of spirit possessing people in order to take them back to a cult headquarters to be used for dark rituals and the like. However, any monster I've come across that can possess people is CR 4 and above, and this would be the party's first major encounter. Is a ghost a viable encounter for 4-5 1st level players? 

I might end up just lowering the hit points and damage output of a ghost, but wondering if anyone would have any alternative ideas. 

Easiest thing you could do is just staple Possession onto a CR-appropriate monster like a Specter or a Poltergeist.

I think the big problem will be all the various resistances and immunities such creatures will have. I’d just delay introducing it a few levels. Maybe hint at what’s going on, make the PCs think it’s just some bandits kidnapping people. Then they go fight the bandits, but the kidnappings continue. But along the way they’ve notched a couple levels, so now they can deal with the real threat. 

How about using a "spooky" doll. Use the Scarecrow as a base creature, they are spirit powered construct so you change its size to tiny and its Terrifying Glare ability to Possession and then the cult can just give it away and it then comes back to them once it successfully possesses someone. It can be found through a use of Detect Magic as it radiates Enchantment magic and has fire damage vulnerability which is the most common so it is quite easy to dispose of.

I'd probably reduce damage to 2d6+3 and hit points to 5d8 (22) or 6d8 (27). However, a carrionette is a possible alternative monster.

I just want to caution the use of character possession as a vehicle to move the plot along.

First of all, if you're pitting the party against it in combat, they could simply defeat it before it gets to possess anyone. Secondly, if you do possess someone, that player has now lost all agency. They basically become a passive viewer while everyone else still gets to play. And now you're railroading in the most extreme way possible - not just telling the party they have to go somewhere but physically dragging a character there without any input from them at all.

I would encourage you to lead the party to the cult in some other way. It's probably something they'd want to take care of anyway. At the very least, have an NPC be the one possessed and let them follow it. If this is a cult doing sacrifices and such, they probably are regularly doing this already.

Rereading this, it seems you might not intend for PCs to get possessed, you just want something that possesses. I would encourage you to explore the idea that you can have a monster do anything you want outside of combat. You don't need a monster to explicitly have a combat possession power to possess an NPC. Whatever you say, happens. It just needs to make sense in the context of the game world.

The idea isn't for the PCs to be possessed, it's instead up to the players to prevent others from being possessed. The creature is possessing innocents for the cult to use their bodies for other rituals.

Quote from JovianDisaster >>

So I'm in the process of making a campaign, and one of the plot points is some kind of spirit possessing people in order to take them back to a cult headquarters to be used for dark rituals and the like. However, any monster I've come across that can possess people is CR 4 and above, and this would be the party's first major encounter. Is a ghost a viable encounter for 4-5 1st level players? 

I might end up just lowering the hit points and damage output of a ghost, but wondering if anyone would have any alternative ideas. 

 It's viable depending how you play it.

If engaged in combat toe-to-toe the party almost stand no chance and most likely get TPKed after dealing little damage to it if any, thus abruptly ending the campaign. Its Whitering Touch has the potential to outright kill 1st level characters.

If run wisely, the ghost can lure the party using intimidation and Possession after withering some plants to convince them its in their best interest to follow it. even using Horrifying Visage, to frightening and aging a long the way.

Another thing to consider is if the ghost succeed at taking the party back to the dark ritual place, what it does after, especially if you plan oen or more combat encoutners with the party there. The ghost is just not a viable enemy to fight to the death for level 1 party. But it could want them to do something specific there so it can lay to rest or avenge it and could even help the party accomplishing some task.

The last ghost i used in my GREYHAWK campaign started at level 1 when a party member found a tiny mechanical spider trinket that moves about when it’s not being observed, after which it began tourmenting the character's dreams, then started to see object being moved, message written in dust or earth, and finished by possessing it to lure it home. It was the spirit of a little girl who had been secretly killed by her stepdad and burried under a tree and told her mother she had run away. The mother hardly believed her beloved daughter would have left her young brother whom she adored. First dream and visions involved choking, earthy taste, being entangled by roots, spiting earth etc... then visions more clear of the final scene etc.. And eventually the party confronted the stepdad, killed him, unearthed the girl's body and helped the mother and son make proper burial for the daughter, along with her favorite spider toy.

This occured during months of play and the ghost never really engaged the party in melee combat but it was a nice story arc.

How about more of a Freddy Kruger/Nightmare on Elm Street vibe?

Ages past the townsfolk vanquished a Night Hag, the Night Hag is now stuck in the "dream dimension" and is implanting suggestions into the Dreams of the townsfolks descendants that makes them sleep walk to a secret grove in the woods where the descendants of the night Hags former warlocks/coven sacrifice them to hopefully empower the Night Hag to return to the material plane.

This way you give the party a look at a more powerful creature and they just have to fight off a coven of low level flunkies and it could set up a recurring villain for the future.

Carrionettes are great for possessing villagers. The live to replicate themselves and so you can end up with an entire village of people possessed by demon dolls. Now all the villagers will act super creepy because they aren't people, but demon dolls walking around in people bodies. Maybe the party spies a doll that's gotten loose and trying to find it's real body so it can swap back at some point. Now if the party ends up having to fight, they're fighting common people and perhaps allowing the dolls to attempt retaking their bodies back. And who started all this? Why the witch cult leader hiding out nearby. But why? That much is up to you.

Just my idea :)

you can fight a ghost at level 1 if you give the players some items to even the playing field. have them dig up amulets to ward off possession so the ghost can waste a turn trying that. when it can't possess on the first one or two tries, it might back off in frustration which allows you to show the monster without every showing becoming a deadly battle.

you might even tell them it's a weak ghost and they could use a significant amount of salt to create barriers on the ground it will not cross. probably not enough salt to protect every house and they can't circle it in during a fight. but, they might get clever and try boxing in someone they found wandering in a trance: up to you whether possession is the one way across that barrier or if that can halt them until daylight comes to (temporarily!) banish the ghost. sometimes there's a lot of fun to be had in 'trying things' in the middle of the story.

and you can always pair them up with a well-meaning but shy and frightened priest with some form of Sanctuary and/or Turn Undead.