The PS4 does not have an AV output. This means that the PS4 can only be used with a HDTV that has a HDMI connection. SDTVs do not contain a HDMI connection therefore the PS4 cannot be used with an SDTV.
There are devices that you can buy to get around this limitation, but doing this is not reccommended. High end PS3 games were often hard to view on an SDTV due to the large amounts of detail in the game and limited amount of pixels on a Standard Definition TV. Since PS4 games have higher levels of detail it will make the problems that PS3 gamers had with SDTVs even worse.
A Lot is gained from playing games in HD. If you are planning to buy a PS4, make sure you also have a HDTV otherwise you are wasting your money