In the context of pre-Mauryan history, consider the following statements:

1. The Mahajanapada period is referred to as the first urbanisation.

2. During this period, the political centre shifted from Indo-Gangetic plains to lower Ganga valley.

3. The 16 mahajanapadas find reference only in Buddhist scriptures.

4. Among the mahajanapadas, rajyas were monarchies and ganas were republics.

Which of the above statements is/are not correct?