Combine first 5 ingredients in large bowl. Season with salt and mix well.
Form beef into four 3/4 inch thick patties approx same size as rolls.
Prepare barbecue (high heat). Grill patties until cooked to desired done-ness, about 4 mins per side for med-rare. Grill rolls cut side down until just warm, about 30 seconds. Spread bottom halves of rolls with sour cream.
Top bottom halves with patty. Place tomato slice, then dollop of guacamole atop beef. Top each sandwich with remaining 1/2 of roll and serve.
Typed by Sherree Johansson