Font #4 hell... The early versions of Excel allowed only four fonts, and their indexes were 0-based. Later they decided to increase the font count, but switched to a 1-based numbering scheme. This means that Font #4 does not exist in an Excel xls file. fpspreadsheet, of course, does not have this gap...
While the gap is corrected at most places in fpspreadsheet the issue still existed on the SST (shared-string-table) which had to be introduced recently because non-SST strings are truncated after 255 characters.
Life is hard.
Please have a look at the current revision.