Is there a way to make a newer version of SpeedTree work for Sims 3, or where can I find the 4.2 version of SpeedTree?

Here's a recent thread that might help. https://modthesims.info/showthread.php?t=695257

I want to create custom SpeedTree foliage. Is there a way to convert an OBJ file to a SpeedTree file? I didn't understand what was meant in the thread you linked.

I have no clue, sorry. I just saw that thread and hoped it would help.

SpeedTree isn't a typical mesh format.
It's more like a set of instructions for drawing a mesh dynamically - somewhat similar to the difference between vector and raster graphics. Parameters are entered in the SpeedTree software that define the shape of each segment of the tree, then the engine assembles them. In short, the original software is required and there is no jumping from .obj to .spt (but you can go the other way) in the 2008 version EA used.

These links are about as far as the TS3 community has ever gotten with manipulating .spt directly
https://acquiresimoleons.blogspot.c...e-recolors.html
https://www.tumblr.com/aminovas/180...s-for-ts3-hello
https://potato-ballad-sims.tumblr.c...utorial-is-very

I was able to find a working 4.2 archive here: https://archive.org/details/speed-tree-cad-4.2
Without EA's original development tools or an S3PE plugin for SPT, it's not an easy nut to crack.