The only need to split the cable at the sub is if your sub's amp is having trouble "waking up" if you run it in auto mode. If you do decide to use a splitter, be sure to recalibrate. As to the cable itself, you want something with enough shielding to not pick up interference, but other than that, it's one of the least demanding cables in a typical HT setup.
The only thing I would add is that a sub cable is usually a long run, and something shielded is important. I have a 6 meter S-Video cable run to my TV, and I got the best Dayton S-Video cable, otherwise, it was awful.
Of course Monster is going to say it is important. They want to sell their high priced cables. Audio signal is totally different from video. Video being much more suceptable to magnetic interference, thus the need for EMI sheilding. That's not saying that you don't want to use a shielded cable for you sub, just that it is less important. It's the old belt and suspenders scenario. Be safe, just do it. I did this on the cheap by using some left over Monster Video cable for my sub. It works fine.
I agree. For sub cables I use a typical left or right analog audio cable... just some el cheapo for my downstairs setup. For my main setup I use a Monster cable run that is some 20 feet and didn't cost more than about $22 after tax.
As for my digital audio for my DVD player... I use a simple coax. video cable to my reciever, for downstairs and a $30 monster wire for my main setup.
All in all I would say the monster sounds a little better compared to the $5 cable but honestly monster cables aren't worth much more than there low-end. After that I think they just make the product look pretty.
As the owner of many monster cables I would say that they are defiantly not worth the price! When I bought them around 3 years ago however I did not know better and bought into the “monster hype”. I mean they are quality cables, but you can get the same (maybe better?) for less money. Anyway I am using a 24 foot “MonsterBass 300” cable, I like it and it has a high quality look/feel to it, but was it worth $60, IMO no.
Four feet? Well, you can't quite run a straightened coat hanger that far but you don't need too much more than that
If you're looking for sub (or other) cables in a variety of sizes you could start at someplace like Accessories4Less ( http://www.ac4l.com ). Please don't take that as an endorsement or anything like that - it's just one place to start or level-set for some sale and clearance prices on cables and stuff like that. As all the wise folks above have already said, you certainly don't need the most expensive stuff in the universe. If you start on some searches you can probably also find tons of references on decent SW cables or even DIY.