Slate are as usually very quiet...no one has details we only know it's coming soon.
The only thing we have is that Slate himself posted on his blog that 4.0 will give one hell of a run for their money "other" drum software, even those with "superior" in their name. His pun, not mine
Upgrade pricing will most likely be the same same in the end as if purchasing new or at least very close. Personally, I'd wait...Slate is going to be flooded in the first few days and you won't be getting you 4.0 license the same day you order the upgrade for sure.