It may just be me but I swear that Vegas once allowed me to drag and drop TIFF image files to the timeline. I upgraded to 4.0e and now it doesnt seem to allow it any more.
Are you sure you can get Vegas 4.0d to accept tiff files. I can't, 4.0d tells me the format isn't accepted. I just use .png format with no problems. Creat them in Adobe Photoshop.
Vegas uses QuickTime to handle TIFF files. You have to do a complete QuickTime install to get the authoring components and not just the player. If you install 4.0e into a different directory than 4.0d then you may have to reinstall QuickTime for 4.0e to use it properly.
TIFFs are still working just fine for me under 4.0e.
You might want to try re-installing Quicktime as Vegas uses facilities provided by Quicktime when using TIFFs. I'd guess that something has happened to your Quicktime installation rather than this being anything directly to do with V 4.0e.
When you re-install Quicktime (the current version is V.6.4) choose a custom installation and tell it to install everything to be certain of getting all the bits Vegas needs.
Probably not much point in using tiffs any way since my digital stills are 18 mb in tiff format. VV simply cannot load up that many lossless file types. Perhaps PNG is the only way to go here.