[5] Continuing north, FM 400 crosses into Hale County, approximately 9.0 miles (14.5 km) east of Abernathy.

[6] It has a concurrency of about 2.0 miles (3.2 km) with FM 37 and passes through the farming community of Happy Union before entering Plainview, where it is known as Date Street.

[7] The highway crosses US 70 and then briefly turns to the east along 24th Street before resuming its northward journey.

[1] On July 9 of that same year saw an extension northward, into Hale County and to a junction with FM 54 west of Petersburg.

The route designation was amended in 1965 after the US 84 freeway bypass south of Slaton was constructed; the old location of US 84 was renumbered Loop 251, which would eventually be redesignated as Business US 84 in 1991.