The other guys are correct.
You can record simultaneously to a hard drive with as many TV tuners that you have in your system.
Some cards are dual tuners; that is, on the card the tuner chipset has two independent tuners. Most dual tuners are analog tuners, not digital.
Most tuner cards are single tuners. So, you would need four tuner cards.
Some cards have an analog tuner and a digital HD tuner, which means that you can record one analog and one HD channel at the same time. If you have two such cards, you would have four tuners for four recordings.
Watch out because, generally, you can't put two of the same tuners in the same box. For example, ATI"s HDTV Wonder has an analog tuner and a digital HD tuner. I quite sure that two HDTV Wonders will not run on the same box.
You could add a different brand like a Dvico FusionHDTV, which also has an analog tuner and a digital HD tuner on the same card. So, you'd have a n ATI and a Dvico doing the four recordings.
The thing here is that the tuner chipsets on the cards should not be the same because the drivers will be the same, and if the drivers are the same, then it won't work. Except if the chipset makers designed the drivers to permit it to work.
Microsoft's Windows Media Center Edition 2005 supports two analog tuners and two HD digital tuners. So, you can record four as the same time.
It is important that the analog tuners are hardware MPEG encoders for WMCE. Hardware encoder cards do all the heavy lifting instead of the CPU, and the results are better quality. Digial tuners record directly to the hard drive and hardly any processing is done by the CPU.
Anyway, that's the short of it.
Lastly, you must have as many PCI slots as you a have tuner cards, unless you use a PCI Express x1 tuner like Powercolors T55E P03 Theater 550 Pro.