A855 - 5 Speed

If it was my trans...I would take it to my friend who has 40+ years experience in gearboxes. He and I would open it up and find out why it's not staying in gear, and if the issue were fixable*, we would fix it. There's no point in continuing to hope the sellers can fix it - they've shown they are out of gas. You need a fresh set of eyes on it.

*I'm kinda wondering if the issue is manageable or unfixable. For example, if they located two of the shafts not in parallel, it might be an issue that only gets resolved by going to a new case, etc. It's interesting...4th gear is usually and most likely the gear when the least is going on as it is direct 1:1 drive.