The Americans were rarely challenged in the tournament's opening stages and they carried their momentum to a ninth consecutive win.
The 31-year-old Donovan, who took a four-month sabbatical around the start of the year, rallied the team with a second-half goal and three assists.
He set up Clarence Goodson's opening score in the 21st minute before Joe Corona gave the Americans a 2-0 lead just eight minutes later in front of an approving home crowd.
El Salvador's Rodolfo Zelaya converted a penalty to trim the lead before halftime but the US broke the match open after the break.
Eddie Johnson and Mikkel Diskerud added goals aided by Donovan, who netted his own score late.
There were fewer fireworks when Honduras beat Costa Rica later in the evening at the same at M&T Bank Stadium in Baltimore.
Andy Najar headed home from Alexander Lopez's cross in the 49th minute to break the deadlock in what was otherwise a dour defensive battle.
Costa Rica controlled much of the possession but could not get the ball past Honduras goalkeeper Donis Escober.
(Reporting by Jahmal Corner, editing by Nick Mulvenney)
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