Grid Framework version 1.7.4 has been approved by the Asset Store team. This release brings official support for Unity 5, so feel free to upgrade. Unity 4 is still supported, thanks to the changes to the Asset Store I can now upload separate packages for different versions of Unity.
There are no API changes, so it doesn't matter whether you want to use Unity 4 or 5. The only real difference between packages are the example scenes. The formats Unity 5 uses are different from 4, so a package created in Unity 5 would appear broken in 4.
