I don't know how well this works compared to the other available tools, but I use Digital Media Converter (v2.7) to convert from various formats, including avi, wmv, dvr-ms and mpeg-2 to mpeg-4 (among others). It's not free, but I recall that it was also not expensive.
In fact, I use this tool a lot to convert my recorded TV shows that were saved using MythTV and MCE and upload them onto my IPT.
For video-editing, I also have many tools (Pinnacle Studio, Nero), but I use Windows Movie Maker to edit and save into wmv format for further conversion into mpeg-4 by Digital Media Converter.
One option for folks is to use the Plextor ConvertX usb-based product; the hardware has the native ability to record/encode directly into mpeg-4, so no conversion should be necessary. This works for Mac as well as PC, but you need different software (drivers and the PVR).
NB: I don't have the ConvertX product; I am only quoting the literature...
EVP