Are the Series 4 Iwatch 44inch bands compatible with Series 3 42inch watch?

Yes

<Change your Apple Watch band - Apple Support>

Make sure that you use a band that corresponds to your Apple Watch case size. You can use a band that's designed for Apple Watch (1st generation), Apple Watch Series 1, 2, and 3 with Apple Watch Series 4 as long as the sizes are compatible. Bands for 38mm and 40mm cases are compatible with each other, and bands for 42mm and 44mm cases are compatible with each other.

Yes, it is, and the old bands 42mm work with the 44mm Series 4 Watch - Forward and Backward compatibility. In my opinion, an excellent thing that Apple did with the introduction of the Series 4 Watch