Spears and Clubs
By far some of the simplest and easiest weapons to use and make, spears and clubs have been used as weapons since the dawn of Man, and for good reason. If your creatures have four arms, they can use all of them on a spear and deliver a very powerful strike. Or a spear can be used in just one hand if needed. The same goes for a club.
The spear would likely be your best choice, you can use it from a distance and all you need to do it thrust it towards someone. Spears would also be good against people who were in trees as well, they have a long reach so you can easily stab someone in the trees. Pikes may also be an option, they’re basically just longer spears, may be good for very high trees.
In reality, any melee weapon can be used effectively, you may need some way of keeping yourself attached to the tree, maybe via a harness or just using your other limbs. Here are some videos demonstrating how a four-armed creature may use melee weapons:
War darts, called plumbata by the Romans, were basically darts that were thrown at your enemy. They had a weight on them to give the dart a bit more stopping power. Your creatures could carry these, throwing them at targets if they are on the ground and dropping them on targets if the creatures are in the trees. Similar weapons were used by WW1 pilots, dropping darts onto planes below them. A more crude option is to just use rocks.
Bagh Nakh or Shuko
Meant as tools for climbing trees, these can also be used as effective weapons. They are often called tiger claws because of their shape and how, when you strike with them, its like you are clawing your target like you are a tiger. These would also fit well into the theme of these creatures climbing trees, you could say its very tree-matic.
An unconventional weapon to be sure, it is primarily a tool meant for climbing or attaching an anchor a far away point. However, being made of steel, if you got hit with one of these i don’t think you’d be walking away. You may also be able to drop it down, hook someone under the chin or arm and hoist them off the ground.
Again it fits very well into this idea of the creatures climbing trees, it would not be ideal for tree-to-tree combat though as it would likely get stuck on all the branches. Although, that could be useful for shaking branches to make them fall. You could also hook it behind a leg and pull them out of the tree.
Handsaws or Wood Axes
Whilst not effective weapons per se, you could easily use them to kill someone. Simply saw or chop part of the way through some large branches, when your enemy steps on it the branch will break under their weight, causing them to fall to the ground and break their bone. Or you could cut part way into the whole tree and fell it when they are in its branches.