I have a 2015 IS250 fSport and I would like to find out if wheels from a 2017 A4 Allroad?

 

No, why would you do that? Different bolt patterns,

 

no.

They wont fit

 

Technically you could mount them but:
Audi bolt pattern is 5x112 while Lexus is 5x114.3
You will need "Wobble Nuts" also called PCD Variable Nuts.
You'd need to find out whether Audi wheels are "Ball Seat" or "Cone Seat" then buy proper wobble nuts.




Also Audi center bore is 66.5mm and Lexus is 60.1mm so a set of 6.4mm centering hub rings are needed.
2017 Allroad uses pretty low offset (ET) on their cars, almost as low as BMW, your new wheels might stick out past the fender. But who knows, with right tire combination this might actually work

https://www.wheel-size.com/size/audi/a4-allroad/2017/