I'm currently working on my first 4t3 conversion, and when I put it in game I noticed there's some distortion happening on the pointer fingers (nowhere else). I thought maybe I accidentally did it in Blender, but I checked and everything looks normal. Does anyone know what could be happening?

Looks like stray bone weights. You can try using MTK to transfer bone weights from a known good reference mesh, or in Blender, cut off and replace the hands.

Interesting... I just looked at the reference mesh I used (afBodyAirForceRecruit) and it has the same problem. I wouldn't have expected that from an EA mesh! I just found another reference mesh and that fixed the problem, but can you still explain cutting off and replacing the hands in Blender? I don't think I know what that means exactly and it might come in handy (no pun intended) at some point. What would be the steps for that?

Something I like to do is to use the original nude geoms (by cloning afNudeTop + afNudeBottom, or the relevant ones for the age/gender you're working with, and exporting the highest LOD) for adding the body pieces to the clothing because they already have their bone assignments vs using .obj body templates.
Then you can use the 'Replace only empty bone assignments' option in MTK when you're copying bones from a reference, so that it only gives assignments to the clothing and doesn't touch the body.

The limitation is that if you're following an existing tutorial they often predate the GEOM tools and expect you to be working with formats that don't retain bones, so you'd have to shoehorn in where to do this independent of instructions given. The 4t3 tutorial linked in a previous thread for example, still uses Milkshape- a program that I wouldn't let touch my CAS projects even with a very long stick

Yeah... I was able to get away with not using Milkshape this time (I think, I'm not quite in the clear yet), but I couldn't figure out how to make it work without using TSRW and OBJs/WSOs. I'd love to do it purely with GEOMs but I think I'm still too much of a beginner to know what exactly needs to be done for everything to work properly—mainly how to create a base package, because I tried the method of making a clone with S3OC and then replacing everything in S3PE, but then that leaves those extra XML files referencing the original textures that I didn't know what to do with. When I tried deleting them I was unable to use CASt in game. There's also the fact that auto-creating morphs in MTK is SO much more straightforward with WSOs, but maybe there's an easier way to do that with GEOMs that I'm not aware of.

I do know of this tutorial on avoiding Milkshape and I'm guessing you could modify it to work with GEOMs, but I don't think it was made with beginners in mind as it doesn't go into a whole lot of detail for each step. Like... I don't even know what seams are, let alone why I need to fix them lol. Same with aligning normals. I will probably make another thread at some point to ask for more clarification on everything because I'd really like to understand all technical aspects! For now I'm stuck blindly following tutorials and just hoping I'm doing the right things.

Thornowl's tutorial is very useful, but definitely only covers the Milkshape alternative steps and otherwise expects you to be following older tutorials. I think further updates to GEOM Tools may have made things even easier since, it just hasn't been documented yet.
I personally learned to start doing things manually with .geom from Lyralei's comment on that post.

In the end, I do it almost identically to the hat-hair method from before. Importing into a base, re-cloning, CTU for presets, etc.
But the TSRW / wso methods for clothes are fine and well established, it's less of an issue vs hairs that largely lacked any documentation. The geom plugin is just great to have on hand regardless of what method you settle on, especially when fixing bones.

Hand bones are very fickle, and position of the hand mesh crirical. There is very little distance between finger, and if there is any variation from the exact EA mesh, bones transferring and mixing between finger is probable. I don't know if you used TS4, or TS3 hands. I generally use TS3 hands, and other skin parts in 4 to 3 conversions because of bone and skin texture alignment.

In full body oufits, be sure to have 59 bones. Not sure if Blender or TSRW adds in a Root_bind, so really, 60 is max. Otherwise, use a 2 group donor. I often use cocktail dress or halter dress.

Geom holds bones, object doesn't. A situation where hand bones do as your did, import the mesh to Blender, and cut off the bad hands.

I go 1 or 2 rows of faces into the wrist area. Import a mesh that you know has good hands, perhaps a nude top from s3oc. Remove the arms and body, leaving the hands with same wrist faces.

Then, join the meshes. Select all vertices and merge. In Mesh tab, Mesh> Normals> Average> faces or corner angle. Smug Tomato has instructions for seam splitting on the Geom Tool page for whichever version of Blender you use. 2.8x has seam fix on export. 2.9x does not. After recalulating normals and exporting from 2.9x, you can use MTK to fix seams. Or, data trnsfer before exporting.