Southwest does not offer stop-overs (stop over 4 hours) so it's not an option on the fare you bought. Although some suggest pretending you felt ill/got lost in airport etc and hope they let you take the later flight for no extra charge. Otherwise, the price is SLC-PHX & PHX-SLC. It works this way with most if not all airlines as stop-overs on discount fares are not very commonly offered on domestic itineraries.