Do not delete the other parts (part01, part02, part03, part05…). Fixing part04 is usually sufficient to recover the whole.
If the archive is for a video or a set of photos, you can often save most of the data even if part 04 is "unfixable." Right-click the of the archive ( part01.rar ). Select Extract files... (not "Extract Here").
Choose a destination for the repaired file and select "Treat the corrupt archive as RAR."