Amad Al Hosni got the opener seven minutes before half-time, and two goals in the space of four minutes made sure of the points late on.
Ibrahim Ghaleb made it 2-0 with a penalty in the 77th minute before an own goal from Husein Al Shuwaish finished off the visitors.
The win puts Al-Nasr six points clear of Al Hilal, while third-placed Al-Ahly are a whopping 18 points off the pace, despite a 3-1 win over Al-Nahda.
Sadick Adams scored for the visitors in the 28th minute to cancel out Aqeel Al Sahbi's opener moments before, but Al Sahbi added a second in the 54th minute before Marcio Mossoro made certain.
Al-Fateh were 2-1 winners over Al Shabab with goals in either half from Bader Al Khames and Gomes Elton, the latter a penalty, putting them in control before Fernando Menegazzo got one back 12 minutes from time.
Al-Faysali and Al-Orobah shared the points in a 1-1 draw.
Abdulaziz Falath had put Al-Orobah ahead after just seven minutes but Rayan Belal levelled for the hosts two minutes before the break.