Final Draft 10 / 9 can open scripts written in FD8 / 7 / 6 / 5. When scripts written in 7 / 6 / 5 are opened in FD10 / 9 they will automatically be converted to the FDX file format.
If you have Windows, you will need to enable Final Draft 10 / 9 to see FDR files. See I have Final Draft 9 / 8 for Windows and it doesn't see the scripts I wrote in an earlier version of Final Draft.
Final Draft 8 can open scripts written in FD10 / 9 but some metadata -- NOT SCRIPT CONTENT -- will be lost.
Final Draft 7 / 6 / 5 can open scripts written in FD9 provided the writer using FD9 has saved the script as a .FDR file. Go to File > Export (FD9) or File > Save As (FD8) and choose Final Draft 5-7 as the format. On Mac this is the Format menu (shown) and on Windows it's the Save File as Type menu.
Final Draft 10 cannot save a file as FDR.
Final Draft 4 cannot open scripts written in FD9 / 8.