Nintendo fans should worry if Sony deliver on the cloud and social aspects, that's pretty much it. Game-wise? Unless you're a dudebro, then the only fans that should be worried are the XBOX fans. Nintendo have always been different.
Yesterday's Tsar Bomba dropped by Sony into the gaming world is still very much a radioactive pit of baron wasteland. Any collateral damage etc is yet to be measured. We don't know too much about the system yet as it stands, but Sony's stance is to go the Clarkson route, with POWER in abundance. However, the PS4, for me, is a Sony-branded PC with features 'politely borrowed' from other devices. That's not to say that it's a bad thing, especially with Nintendo themselves being at the forefront of innovation, but there comes a time where Sony must push the Vita, lest it's doomed to failure.
Someone I know believes the PS4 will make the Wii U obsolete once it hits the shelves, but they seem to have completely overlooked how bad Sony are doing on the whole. Sony's stocks as a business are virtually worthless, their electronics division has taken a serious battering from both a stagnating Yen and rival companies elsewhere selling the same kind of technology for cheaper (e.g. LG, Samsung). Their Walkman line has all-but-vanished thanks largely to the iPod, and likewise their phones are suffering (much like HTC is) due to the prevalence of other Android-devices and the iPhone. The PS3 only achieved moderate success, and the PSP a little less than that. The Vita is failing even as we speak, so this truly is the last throw of the dice Sony have. They've literally gone all-in, so it's do or die for them.
If the PS4 fails, Sony are dead. There is no other way of putting it. Nintendo cannot tank the industry on its own and the Wii U will most likely continue to suffer due to media perception, which have made it somewhat clear that they want Nintendo hardware to die and for Nintendo to become a Third-Party developer. Why I have no idea. It seems mighty unfair. Anyway, the media seems to believe the big fight will be between Microsoft and Sony for the Eighth Generation, so let Nintendo slip quietly into the shadows.
One thing is for sure, the PS4 won't be cheap upon release. Don't be surprised if the price tag is upwards of £500.