English Standard Version
fully convinced that God was able to do what he had promised.

American Standard Version
and being fully assured that what he had promised, he was able also to perform.

Berean Study Bible
being fully persuaded that God was able to do what He had promised.

Douay-Rheims Bible
Most fully knowing, that whatsoever he has promised, he is able also to perform.

English Revised Version
and being fully assured that, what he had promised, he was able also to perform.

King James Bible
And being fully persuaded that, what he had promised, he was able also to perform.

World English Bible
and being fully assured that what he had promised, he was also able to perform.

Young's Literal Translation
and having been fully persuaded that what He hath promised He is able also to do:

Romans 4:21 Additional Translations ...