In 1944, a fourth line was constructed and it absorbed the branch of line 2 between Goya and Diego de León in 1958, a branch that had been intended to be part of line 4 since its construction but was operated as a branch of line 2 until the construction of line 4.
1944 tillkom en fjärde linje som byggdes samman med linje 2:s gren mellan Goya och Diego de León 1958, en gren som hade varit tänkt att bli en del av linje 4 men som hade använts som en del av linje 2 till dess att linje 4 byggdes.