Sliding door handle
Thanks for posting. The handle on the right hand side in your photos is a 'dummy' handle that doesn't interface with any lock in the door, is that correct? Whereas the handle you are replacing interfaces with a lock inside the left hand panel, is that correct? If so, I'd like to see the lock inside the door to be safe, but I believe a handle set like our 82-089 or keyed 82-086 could work for you. Note that the hole spacing on these handle sets is 3-15/16", but this is based on the mounting holes on the pull, rather than the holes you are measuring which I believe are for a pull to attach to the base. See the image below for reference. Compare with your old hardware and see if you think this could work for you.
I've never seen this handle set made with a 4" hole spacing. I'm pretty confident that your spacing is the same as ours at 3-15/16". Make sure to measure from the center of each hole, not their edges.
As for being left or right handed, the 82-089 and 82-086 can work for either side interchangeably. You can use the 82-086 for the "main" handle, with key lock, and the 82-089 for the "dummy" handle. Just note that the 82-089 is technically not a real dummy handle, it just lacks a key lock. We don't sell dummy handles of any kind so this is our sole alternative.
It looks like you're trying to put the pull handle from the Legacy handle set on your old base plate. That's not going to work, unfortunately; my previous recommendation would only work if you replace anything, including the base plate.
We don't sell these pull handles separately, and we also do not have anything as long as your old base plate requires.