If you like to play games, there are many affordable 24" 1080p monitors available. Just make sure they have a HDMI port so they can connect to your PS4.
I use a 27" 1080p monitor and I am very happy with it. 27" is just big enough that you dont have to sit too close and just wide enough that it doesn't completely fill your view.
If you can find a good deal on a 24" 1080p monitor, go for it. If you can find a good deal on a 27" 1080p monitor, also go for it, you cant go wrong.
Also remember to check the response times. Response time is the time it takes for the monitor to update with a new frame or image. Anything under 5ms (milliseconds) is fine. However, because you are gaming on a console I would recommend anything under 10-15ms...