Could a PPL fly a 747?
It depends on your definition of 'a PPL'. There is no reason to believe that a person could not fly a 747, or any other aircraft, purely by virtue of the fact that he or she holds a PPL. The licence that one holds is no measure of experience or ability - for example, I now hold only an NPPL (arguably a lower status of licence than a PPL) and I can fly, and have flown, a 747.
The practicality, however, is slightly more complex. Before commencing a B747 type rating course (assuming that it was a first MPA type) you would need 100 hours PIC, an MCC course, a valid MEIR and a pass in the ATPL exams. You would also need to find an approved TRTO that was prepared to take you on as a customer and a B747 operator that was prepared to allow you to complete the mandatory flight training in their aeroplane.
The answer to the question is, therefore, yes in theory but probably no in practice.