The last I knew, it was planned for distant releases. I've not heard much about 4.0 and have no reason to believe it will be in 4.0.
I've been looking forward to this feature for years as I hope it will ease common modlet installation issues and allow for easy resolution of modlet dependencies. Another person on these boards mentioned they thought they read the feature had been cancelled in favor of some other solution, but I've not been able to confirm that.
I'd love an update on this topic too, if folks in the know see this topic and have news.