Didn't touch 4.0 yet and will not until the dust settles. But I don't expect much to change in the area this mod touches. The only thing might be the energy job output research, as that is the only thing that is even remotely pop-related.
The rest should be fine. Let me know if not, then I'll take an earlier look.

does this work with 4.0?

Theoretically yes, since this mod overwrites techs I would expect all techs in it to appear, regardless of the presence of the original mod. In case of gigas we're talking about more megastructure build speed and resource storage, so it wouldn't be bad.
Didn't test that, but feel free to do so. I would be interested to know the result and would even add that to the description.

Does this work without Gigastructural Engineering?