Gently shape beef into 4 balls, shaping just enough to keep together (do not compact). In a large skillet, heat oil over medium heat. Working in batches, add beef. With a heavy metal spatula, flatten to 1/4- to 1/2-in. thickness. Cook until edges start to brown, about 1-1/2 minutes. Turn burgers; cook until well browned and a thermometer reads at least 160°, about 1 minute. Repeat with remaining beef. Remove from skillet; wipe skillet clean., Combine mayonnaise and salsa; reserve half for serving. Spread remaining mixture over tortillas. On the center of each tortilla, place 1 slice cheese, 1 burger and 1 tostada shell. Fold sides of tortilla over burger; fold top and bottom to close, pleating as you go., In batches, place wraps in skillet, seam side down. Cook on medium heat 1-2 minutes on each side or until golden brown. Serve with remaining sauce.