Hello there! :)
Perpendicular lines have slopes that are opposite reciprocals.
So, if the slope of one line is 4, then the slope of the line perpendicular to it would be the opposite of 4 flopped over.
So the number is -1/4.
Hopefully, you find this helpful! :) Enjoy your day!
Feel free to ask me if you have any query.
~An excited gal
here to help