You're worried about a 12 microsecond blocking (my code blocks for 10 microseconds) every four MILLION microseconds.
That's not something you should be worrying about at the moment.
Given that the speed of sound is something like 8.5mm in 25 microseconds, pulseIn blocks for much longer than your delays.
Your revised code still does not work, and you have demonstrated that you still need to learn the difference between milliseconds and microseconds.