Final Draft 10 / 9 / 8 can open scripts written in FD 5, 6 and 7. When they're opened in FD10 / 9 / 8 they will automatically be converted to the FD9 / 8 file format (.fdx).
Final Draft 7 / 6 / 5 can open scripts written in FD9 / 8 provided the writer using FD9 / 8 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 scripts as FDR files.
Final Draft 4 cannot open scripts written in FD9 / 8.