Indiana's 4th congressional district runs from the western Indianapolis suburbs northwest to the Illinois border, containing the city of Lafayette. It is solidly Republican: Trump won the district by 34 points in 2016, though McCain only won by 10 points in 2008. Though some of the shift in 2016 towards Trump may be due to the fact that former Indiana governor Mike Pence was Trump's running mate, it is still possible that the district has recently shifted to the right.
Republican incumbent Todd Rokita did not run for reelection in order to run for the Senate. The candidates to replace him are Republican Jim Baird, a member of the Indiana House, and Democrat Tobi Beck, a technology professional and veteran.