Family Tree Builder constantly makes back ups of your family tree while you work on it. You should have a back up of the family tree that you can revert to if necessary.
You can also revert back to Family Tree Builder 3.0 at any point in time if you wish.
Hundreds of thousands of people have successfully downloaded and installed the program and have not had the problem you describe. It is possible that the family tree file you opened somehow became corrupt.
It would be helpful if you could recount the exact steps you took beginning with downloading and installing the new version.
I assume the file was fine when you viewed it in Family Tree Builder 3.0?
What languages are you working in?
Has anything else on your computer changed since the time you installed version 4.0?
It only has 138 people on it. Because I had free membership, I was only able to back up a small part of my tree. If we don't take membership out, does that mean that we cannot back up our trees and years of work can be lost?
If you want to use the option of backing up your family tree online by publishing it to your family site, then yes, you will have to purchase a subscription that will accomodate the size of your family tree.
The 138 person family tree is the only tree on your family site. I'm sorry, but we don't have another copy of your tree.
In Family Tree Builder you also have the option to backup your tree on your computer (File > Backup). This backup can be saved to a CD or other removable media device in the case that something happens to your computer.
You can use the backup option in Family Tree Builder to backup your tree of any size for free. There are no limitations on using this backup option.