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Are creatures with 4 arms viable underground?
Creatures have to be able to climb and eventually dig!
Viable? Probably. Ideal? That could take some imagination to manage.
The traditional four-armed humanoid, with the extra arms directly under the base two arms that come with the 'humanoid' designation, has all of the issues that Demigan mentioned. But those issues do not mean that such a creature could not exist, just that it would probably not evolve from a two armed humanoid that existed in that same environment first.
Giving some distance from the primary arms, the secondary arms could have more range of motion without compromising the upper arms. However, in this configuration, there may need to be a bit more adjustment from stock humanoid to get them to be attached securely.
Another option would be to have the second pair of arms be specialized like Supernormal Step's Akela. Note that I'm not necessarily saying that specific specialization, but the extra arms don't have to be identical copies of the first pair. If they were, for example, claws that could break rocks, the larger, stronger primary arms could then move the bits of rock they broke. There are other specializations that could be useful as well.
That said, Demigan is right about it being complicated to add arms and retain flexibility. I'm not entirely certain about whether or not having an extra two places to grab things is really less useful than having double the strength in just two arms. However, any muscles in the extra arms do also require resources both to grow and to operate, so they do need more than negligible utility to work well.
Well at least we know why they are going extinct... they have 4 arms.
You need all your strength to dig and climb (holding your entire body weight on your arms!) so it's better to focus your muscle strength on 2 arms rather than 4. You are better off using larger arms than 4 arms.
More problems with 4 arms is instability. Human arms are incredibly ingenious with how they can move compared to the torso and still have relatively high stability when moving them. 4 arms would mean that many muscle groups would need to be shared or covering each other with holes in them to allow tendons of the lower muscle groups through, limiting movement and strength of both torso and arms. Add insult to injury in where and how the extra shoulder blades need to be placed on an extension of the rib cage and fastened to its already busy surroundings and these arms will mostly be stuck in one position. It kind of defeats the point of having two extra arms if either both arms move in the same direction each time you try to move one of the arms or if you become a stiff puppet that needs to move its entire body when it wants to grab something not directly in front of it.
