I've seen a discussion about this elsewhere in the forum, but since the solution (importing within PE11) was something I had already done, I wanted to post this to see if I could get a more precise answer so I understand what is happening. Here's the background:
I have footage from two mini-DV cameras, one a true widescreen (but not HD) and the other a standard ratio that simuated widescreen by using only a portion of the sensor.
After some good input from this forum and some test editing, I decided the best approach was to start a new project and import the true widescreen first to allow PE11 to set the project options from it. Then I imported the pseudo-widescreen footage and zoomed and sharpened it to fill the widescreen space (it came in as a standard ratio image, within which was a letterboxed "widescreen").
Everything good so far. I edited the synched footage down to 1.25 hours of cross-faded video and burned a DVD using the "NTSC widescreen" option. On a standard TV, the image was severely cropped on each side. So I tried viewing it on a widescreen TV, thinking that this setting must only work with that, and found that the image was still cropped, but stretched across the widescreen format. So I burned a normal ratio DVD. That appeared as properly letterboxed on the standard TV and as a standard image in the center of the widescreen, with letter-boxing (that is, unlike a commercial DVD I could only get a "filled" widescreen image by using the zoom controls on the widescreen TV).
Why did the widescreen export result in a cropped image? The only real wildcard that I can see here is that the pixel ratios of the two footages were different, but since I started with the true widescreen footage, I assume that PE11 picked up its 1.2 ratio first and ignored the .9 ratio of the other footage. Or maybe I have no idea how this works... ;-) Any comments? (Please!)
It could well be a pixel aspect ratio thing. Though I'm a little unclear on your workflow. But try this:
Open a new project and force it to DV widescreen settings (checking the option on the New Project panel, under the File menu). Add all of your 4:3 footage, scale it up and sharpen it to fill the video frame -- then use Publish & Share/Computer/DV Widescreen to output the footage.
Now, when you add this converted footage to your widescreen project, it will be "true" widescreen and should mix perfectly with the real widescreen footage.