Try the direct download URL http://www.osirix-viewer.com/OldVersions/OsiriX4.1.2.pkg.zip, which AFAICT is for the last version running on Snow Leopard. If necessary you can download other versions (except 5.x and 6.x) by substituting the version number components (in the case above 4.1.2) with others pulled from this version page.

While you’re at it, you may also want to look into Joël Spaltenstein’s UnImportantNotice plugin, which supposedly removes the watermarks OsiriX puts on image renditions of the free versions.

alternaut ◉ moderator