Which version of Fennec did you install? The official alpha release should work on the Evo 4G, and so should today's nightly builds (but the stability of the nightly builds varies from day to day). You can find both on this page:

https://wiki.mozilla.org/Mobile/Platforms/Android

(You will need to uninstall the previous version first if you want to switch from the alpha release to a nightly build, or vice versa.)

There's also a bug in Fennec for Android that causes it to hang if there is too little internal storage space (<35MB or so) available at startup. We're working on fixing this.

I did it three times. I also made sure to clear the data and cache