I'm guessing at this and with the help of a different post: https://www.homebrewtalk.com/f25/calculating-gravity-101641/
I was helped with a similar question earlier today, here's my thought:
4gal(1.070sg) + 1gal('x'sg) = 5gal(1.090)
4(1.070) + 1x = 5(1.090)
4.28 + x = 5.45
x = 1.17, you...