Yes you can upgrade the HDD. This does NOT void your warranty. The main limitations on the HDD are physical size. It must be a 2.5" SATA no higher than 9.5mm. So far the largest capacity available is 1.5T. There is a 2T but it's not available to consumers yet.
These are reasonably priced 1Ts. Look around newegg if you want larger.
Or, if you have the money, get a 1TB SSD, Or cheaper, but still fast, get a Hybrid drive. That would be awesome. I plan to do this, at some point.
i believe this would be the best swap you could make at this point in time, otherwise you would be waiting till around summer of 2014 for this 2tb sshd to come out by seagate....at least with this, you could keep it in your ps4 until the better one comes out, then switch your ps3 hdd with this (if you still have it, which i do personally) and put the better one in your PS4.
once again, this is all stated in my personal opinion, not a demand or a order for you to do it, just a sugestion....
Does it have to be a sata 2 and can the speed be faster the 5400 rpm
What type of hard drive does PS4 use?
PS4 is equipped with a 5400 RPM SATA II hard drive. Users can choose to install a new hard drive so long as it complies with these standards, is no thicker than 9.5mm, and is larger than 160GB.