If you've created new colorways or made scans of fabrics, EQ4 supplies the means to add them to the fabric libraries for use in your quilts. EQ2 is not compatible at all, and EQ3 projects can be brought in, but they can play tricks with colors and other aspects of a project. The move to Windows has not been without its trade-offs, but by and large you should be able to make the transition without losing too much of your earlier work. EQ4 comes with two books. And we have the familiar EQ Design Cookbook, which teaches more advanced techniques in an informal style.
Online help is also very comprehensive and clearly written.
The Electric Quilt Company has always shown itself to be non-defensive and willing to listen to its customers. The results of their having open ears shows very clearly in this new iteration of their program, which incorporates hundreds of suggested improvements from EQ users. Judy, it's here.