I have used an Adaptec dual tuner USB tuner and HDHomeRun (connects over Ethernet) in Windows Media Center running in Parallels. They record just fine, but of course you can't play back video in Media Center, though you can in Media Player. If you have a Media Center Extender to view the video, you can definitely use a Mac & Parallels and Vista or Win7 to record your TV shows.
The only question would be if there is enough disk performance to record 4 TV streams. I only tested if it would work, I didn't test it under full load on the Mac or the Windows side. It's only a few hundred dollars to build a real PC and use it as a dedicated DVR, but a Mac will work in a pinch.
I tested with a first generation 2.92GHz Mac Pro with 9GB of RAM (1GB to Vista) and a dedicated drive with only a Boot Camp partition.