From the Options menu, select General Preferences..., then the Helpers tab. Then click on the Create New Type... button. For Mime Type use either application or image. Each works the same as near as I can tell. For Mime Subtype put sa4. For File Extensions, put sa4,sa3. For Action: select the Launch the Application radio button and fill in the full directory path to your sa4.exe file. Click the OK button and you're ready to view Street Atlas USA files over the web.
Please let me know the results of using other web browsers with Street Atlas USA.
AddType application/sa3 .sa3
AddType application/sa4 .sa4
AddType application/sa5 .sa5
Or you can put the above lines in an .htaccess file in the directory that contains the Street Atlas files.
If you know the equivalent for other servers, please let me know.