C. F. Keil and Franz Delitzsch on Isaiah 26:4

Isa 26:4

A cry goes forth again, as if from heaven, exhorting Israel to continue in this mind. “Hang confidently on Jehovah for ever: for in Jah, Jehovah, is an everlasting rock.” The combination Jah Jehovah is only met with here and in Isa 12:2. It is the proper name of God the Redeemer in the most emphatic form. The Beth essentiae frequently stands before the predicate (Ges. §151, 3); here, however, it stands before the subject, as in Psa 78:5; Psa 55:19. In Jah Jehovah (munach, tzakeph) there is an everlasting rock, i.e., He is essentially such a rock (compare Deu 32:4, like Exo 15:2 for Isa 12:2).