Ksoup,

I'm re-building one of my sites with a fresh 1.5 install and just yesterday I installed IH4:

http://www.promdressesforless.us/

At first my IH didn't work either. But then I remembered two things:

1. Had to turn it on in admin/configuration/images/IH resize images;

2. I ended up having to manually move the various INSTALL files into my respective template folders. I've had to do this before, mostly when switching templates to test. You might take a look at your template files to see if the IH4 files are inside them.

Now IH4 is working and zooms my smallest thumbnails.