usually most gamers will play on the ps4 from his couch or bed, not in front of the desk using a big tv
having said that, you need a monitor with multiple inputs, at least one hdmi for the ps4 and other for the pc or a displayport input
about speakers, if he is going to use the monitor as a single option, he is going to need good headphones or a decent set of speakers, the minimum i would get is a logitech 2.1 speakers setup, something like a z523 or similar, connected to the monitor audio output, that way, when the monitor slects input as pc, sound will come from it, and when the ps4 input is selected, same will happen, i do this on my setup with a pc and a ps3
about the size, depending on the distance, that 27" monitor might not be enough if he plays from his bed or a couch on the ps4
so consider buying a 32 or 40" tv for the ps4 and a 24" monitor for the pc
this idea of 2 instead of 1 works for some, for others as you mention, is a problem in both space and costs of implementation