When you drop your original HD clip into your 4:3 SD sequence in Premiere, you will need to resize the video.  Then use these settings when exporting from Premiere.
 
 

Compressor Settings 
 
Start by selecting the MPEG-2 Program Stream, 15 Mbps and adjust to match the settings in the files below:
 
 
 
 
Once your file has finished, change the extension from .mpeg to .mpg.
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