Sleep tight, stickman!

Generally, four cyberlimbs is a "rule of cool" kind of thing. Both in the universe and from a mechanics perspective, it's rarely worth it to replace all four limbs when you can get less invasive 'ware. If you got your arms and legs chopped off in an accident or something, that would probably make sense ;)

But I'm not extremely opposed to the idea. It could be that someone would get most of their body replaced with cyberlimbs for intimidation reasons—if someone who's 90% machine wants to mess you up, you might be inclined to tell them what they want to know. It almost veers into cyberzombie territory, though. The body can only take so much mechanical punishment.

It just occurred to me that the price of those cyberlimbs would be a good reason for a character to be shadowrunning. You wanna get a hot cyber-bod? Talk to someone in Renraku/the Yakuza/whatever, borrow some money. You'll be in debt for a zillion years, but you'll also be deadly.