Two things are certain:
1) Nobody knows what this new game is
2) It will be good as LDW is making it

I do think it could be VV4 because an official suggestions page is set up already.

That is right!

it's less sure, the official suggestion thread for VV4 was here before VV3 was finished.
It was because they were so much suggestions posted that the moderators had to create that thread to arrange the forum!!

Well there could be other reasons they didn't want us to know...

This is actually pretty standard practice with new games; companies can go bankrupt, loose funding, be bought and sold, or perhaps the technical challenges were underestimated and the task became too daunting. If things fall through, nobody wants to have the black mark of having announced something they couldn't make good on.

Most companies don't announce a game that's only 10% complete unless they have a 99% guarantee that they will have the freedom, funding, and expertise to complete it. Also, they usually wait until they have a reliable schedule for completion and release, so that they can market it properly.

The VV3 suggestion thread had been going for a long while and the next game released by LDW was ... Plant Tycoon.

I am just glad that there is something on the preview page with a progress bar. I have other casual games, but come back to the LDW games more than any. Whatever it is I will probably become addicted to it too.

Yay I saw it too

Given the story line of VV3, I wouldn't surprise if VV4 wasn't already in the works, or altleast being planned out.
Just my opinion.

CFII has stated that they DO plan a VV4, and given that they storyline in VV3 is kind of open-ended, I'm sure they had that part of the story planned even before they finished VV3.

She has also said that they never stated VV4 was their NEXT game, which can be taken either way; either as obfuscation of a direct hit, or as honest dissuasion of a mistaken assumption.