Claudia

The surface sector should not count as one of your 4 free sectors, it does however count as one of your stopovers. But if you were actually stopping in Nairobi it makes no difference. LHR-NBO-surface-Lusaka should count as one stopover but no intra-African segment.
By the way, I don't think you can fly out of Harare anymore when leaving Africa after coming from Europe. I believe that QANTAS no longer fly tom Harare but are awaiting approval to code-share on SAA flights. Why they don't codeshare on their Oneworld partner British Airways' flights I don't know.