Try to find a .veg.bak file with the same name. It might still be a VV3 vesion. If not search your computer for a file with 'autosave' in its name. Check for the details of that file to see if it has the date of your last VV3 file.
This will only work if you have 'create bak files' and 'autosave' selected in Vegas preferences.
If that fails, you should be able to get at least the skeleton of the project back into VV3 by saving the V4 project as a Text EDL and then opening that in VV3.
The format of Vegas's .EDL file doesn't seem to have changed from VV3 to V4 as far as I can see, so you should be able to rescue all the features that are contained in the EDL. Unfortunately the EDL does not contain all the features of the project, only a simplified version, so you will have to do some reconstruction once it is back in VV3. But it might be enough of a start to be of some help.