How would I construct: a square (perpendicular) line from A-A1 through point A4

Here’s what I did. First a drew a helper line (A4-A5) from a4 down on the same angle as A-A1 Then used this line to construct a right angle leftward until it crossed the A-A1 line (ending in an arbitrary length) at A6 Then used the tool ‘intersect line and perpendicular’ to find the crossing point (A7) along line A-A1 Only then I could go ahead and create the square line from A-A1 through A4 (ending arbitrarily at A8) By which time it looked like:

I hope this can be done more easily. On paper this would be a doddle