(Recasts throughout; adds background, share reaction, analyst comments) SAO PAULO, Oct 19 (Reuters) - American Airlines has placed a firm order for four Embraer E175 jets to fly with its subsidiary Envoy Air, the Brazilian planemaker said on Thursday, marking the carrier's second order for the regional jets this year. American's previous deal for seven E175s, announced in June, underscored a U.S. regional aviation market recovering from rough years marked by the COVID-19 pandemic. Shares of Embraer rose as much as 1.2% on Thursday's order news, which analysts flagged as a sign of improving demand and stronger deliveries going forward. "It highlights that the company's efforts to increase its commercial backlog is bearing fruit and also indicates that demand for E1s in the U.S. could be picking up again," Santander analysts told clients, referring to the first generation of E-Jets still popular with U.S. regional carriers.