From the situation you described, A3 is not eligible and cannot touch a forward pass behind, in, or beyond the neutral zone. A1 is eligible due to being on the end of the line, A2 and A4 are eligible due to being off the line.
Rule references:
7-5-6a: All A players eligible by position and number include those who, at the time of the snap, are on the ends of their scrimmage line or legally behind the line...
7-5-13: An ineligible A player has illegally touched a forward pass if he bats, muffs or catches a legal forward pass.