I need to make new, oversized shafts for my cat TEN front idlers.  What material would be best, but can be machined with carbide tooling?  4140?  Thanks.

1) 4140 is good,
2) why not just cold roll shaft, easier to work with,, your not putting many hours on it.
3 why not just replace the bushings

The worn original type bushings can be bored out a bit oversized to true them up. The current shafts are worn and really bent. I can make a shaft with an oversized center area and all will be good for decades.

Use 4140HT or 4142HT that is a little harder and can still be machined than annealed 4140 like you mentioned. I've made several of these shafts standard and over sized over the years. If you are going oversize; Yes you can bore out the bronze bushings but, you may need to take a close look at the two castings that slide on the track frames. I've encountered ones that are egged out and then they will need to be bored oversize. Once you're into those ; you may want to machine the flat area ware spots from the track frame channels sliding and rust.
The original shafts were case hardened and hard chromed and you most likely have two notches(Not the oil clearance notches) worn from the tracks being tightened.