Adam Clarke on Psalms 89:4

Thy seed will I establish for ever, and build up thy throne to all generations - And this covenant had most incontestably Jesus Christ in view. This is the seed, or posterity, that should sit on the throne, and reign for ever and ever. David and his family are long since become extinct; none of his race has sat on the Jewish throne for more than two thousand years: but the Christ has reigned invariably since that time, and will reign till all his enemies are put under his feet; and to this the psalmist says Selah. It will be so, it is so; and it cannot be otherwise; for the Lord hath sworn that he shall have an euerlasting kingdom, as he has an everlasting priesthood.