Drupal 7, Custom Content Type... Does an addon that does this exist? I rebuilding my companies intranet... we have what's called a portal. Within the portal is pages. Each portal page is either HTML, a PDF or a Video.

Right now... I'm planning on creating 3 seperate content types. One with Text, one with a custom PDF Object for loading in browser and one for a video. So content types like:  

PAGE content Type 
* Title 
* Image (Icon for content flagged News)
* Text with Teaser

PDF content Type  
* Title  
* Image (Icon for content flagged News)  
* PDF Object (loads PDF in browser) 

VIDEO content Type  
* Title  
* Image (Icon for content flagged News)  
* Video Field

FILEVIEWER content Type  
* Title  
* Image (Icon for content flagged News)  
* FileViewer Viewer


Notice... a lot of duplication. 

What I'd like to do, and tell me if I'm stupid... is I'd like to have a drop down selector that shows the content type selected, while hiding the rest.

COMBINED CONTENT TYPE
* Title  
* Image  
* Select List (Three, or more, choices: Text, PDF, Document, Video)
* Text w Teaser  
* Video Field  
* PDF Field  
* Fileviewer Field  


It will display the selected content type and hide the others when displayed on the actual node/block. This particular case is simply "Select One type", so I'm not worried about selecting combinations (ie: Text + Video)

I'd just much rather baby one content type... than 3+ content types with near identical layouts.

Question updated. Basically... it's my desire to have one content type instead of 4 (and possibly more). I'm not afraid to make my own custom module, although it would be a learning curve.
The obvious choice for that functionality would be the conditional fields module. It's still under development for D7, but you can check if it already supports your use case, or maybe you could even help the maintainer get the D7 version to a stable state.