I really am at my peak of frustration trying to convert a S3 hat to S4. S4S is way too confusing, but CAS Tools keeps crashing. Then, there are no new conversion tutorial using the updated CAS Tools.

Which version of CASTools do you have there, is that a current one? If so you should report your crashing issue in the download thread.

If you're trying to open a TS3 package in CAS Tools, that may be causing your crashing.

Off the top of my head, you can try:

Open the TS3 package with S3PE and export the TS3 geom hat mesh.
In CAS Tools, use the Mesh Tools tab to convert the TS3 geom to TS4 geom. Do NOT select a reference mesh.
Still in CAS Tools, clone a game hat. Open that clone package in the clone editor tab.
In Mesh Manager, import the TS4 mesh you created above.

If that went okay, you can export the mesh as an MS3D Milkshape mesh and use Milkshape to check bone assignments etc. I seem to remember TS4 uses __headnew while TS3 uses __head, or the other way around, or something. Or not. I'm not at my modding computer but can check later. You'll have to adjust the UV map to locate it in the TS4-standard hat area of the texture. Also since TS3 hat hairs include the hair you'll have to modify the mesh unless you're converting an accessory hat. And of course you'll have to modify the texture by copying the TS3 hat image into the correct location in the TS4 texture image.

If you need more details I'll be happy to work with you.

I did some video tutorials that may help, TS4 CAS Tools Tutorial, How to convert Sims 3 clothes for Sims 4 - Part 1, How to convert Sims 3 clothes for Sims 4 - Part 2