WMV vs MP4 Streaming & Download
Your comparison of WMV to MP4 is a little bit confusing, because you're comparing apples and oranges.


MP4 is a container format, which may contain a variety of audio and video formats.  Most commonly, an MP4 file will contain wither an MPEG-4 Part 10 (aka H.264) or MPEG-4 Part 2 video stream, although it can contain MPEG-2 or MPEG-1 video streams.  There are a number of audio codecs that can be stored in an MP4 file as well. See Wikipedia for more information.
WMV is an actual codec (akin to H.264 or MPEG-4 Part 2 mentioned above), and files that end in .WMV are generally stored in an ASF container (see here).


What this all boils down to, is you need to use a codec that both Mac and Windows can play.

According to Microsoft, as of July 2010, Windows Media Player for Windows XP supports (among others):


Microsoft MPEG-4 Standard Video Codec
Vivo H.264 Video CODEC


Which suggests that it ought to play back the two common video formats contained in .MP4 files.  Note the caveat:


  NOTE: Not all of the supported codecs are installed by default. These codecs will be downloadable when you play content that requires the codec.


So it's possible that your users may be prompted to install a codec the first time they try to play one of your videos.

You'll also want to make sure you use an audio codec supported by Windows Media Player. The most common (but by no means only) one is probably MPEG-1/MPEG-2 Audio Layer III (aka MP3), and this is in the list of supported codecs.

So based on this information, if you use MP4 streaming, Windows users should have no trouble playing back your videos.  Although you'll surely want to test your specific output files on a fresh Windows (and Mac) install before you commit to this decision.