Translation:Biology is my fourth favorite subject.
I know that this discussion has been dormant for a few months, but I would agree that "lesson" -- in a colloquial sense -- is used in English as term for "subject". The context here (i.e. if I would say "subject" or "lesson") would be who I was talking with at the time.
This is interesting, as a non native English speaker I thought μάθημα was used as lesson only in Greek. And not even just colloquially. Hence the 'isn't it' in my comment. At this point I still find 'lesson' in the phrase 'what's your favourite lesson' a bit out of place or a poor translation of 'ποιο είναι το αγαπημένο σου μάθημα', but I see your point. Thanks for that!
I think in that case it would stand, but it'd need another clause, which isn't in the sentence, eg, "biology is my fourth favourite lesson of the day" /"Η Βιολογία είναι το τέταρτο αγαπημένο μου μάθημα της μέρας". With the sentence as it is, I think subject is a more accurate translation.