will a watch band from apple watch 2 fit the new apple watch 4

Yes, they specifically said so during the rollout event. ◄Dave►

The bands from Apple Watch Series 0, 1, 2, 3, and 4 are the same. However, there are 2 Apple Watch band sizes: The ones for 38mm and 40mm (Small Apple Watch), and the ones for 42mm and 44mm (Big Apple Watch), and that size DOES matter, meaning that you cannot use a Small Apple Watch band on a Big Apple Watch and vice-versa.