4:14a–c

For if those who live by the law are heirs, faith is useless and the promise is worthless: Here the word if introduces a situation that was not true. Adhering to the law only does not make someone an heir of this promise. But the “then” clauses (faith is useless…) imagines what might have happened if the adherents of the law had been heirs. Translate this if -clause with that meaning. For example:

if, for example, those who live by the law are heirs
-or-
if ⌊it were true that⌋ it is those who live by the law are heirs

See the grammar you used for this kind of if-then construction in 4:2 (“if Abraham was justified by works, he has something to boast about…”).

4:14a

For: This word introduces an explanation of the connection of the promise to faith and not the law.

those who live by the law: The Greek is more literally “the (ones) of (the) law.” The word by indicates that these people are connected to the law of Moses. It refers to the Jews. They possess this law and live under its rule. Here are other ways to translate this phrase:

those who are of the Law (New American Standard Bible)
-or-
people who have/possess the law ⌊of Moses⌋

heirs: Translate this word as you did in 4:13a, but as a plural.

4:14b

faith is useless: The Greek word that the Berean Standard Bible translates as is useless means “has been made empty” or “has removed the power of it.” If getting the inheritance is based on keeping the Law of Moses, that makes Abraham’s faith useless in regards to getting that inheritance. Here are other ways to translate this clause:

it/that has made ⌊that⌋ faith empty
-or-
that has caused ⌊Abraham’s⌋ believing ⌊God⌋ to be powerless/ineffective
-or-

⌊his⌋ faith has borne no fruit

4:14c

the promise is worthless: The Greek word that the Berean Standard Bible translates as is worthless means “has been made ineffective or powerless.” If getting the inheritance is based on keeping the Law of Moses, that would have made the promise to Abraham invalid. Here are other ways to translate this clause:

it/that has made the promise invalid
-or-
that has caused the promise to be powerless/ineffective

The Greek word here is similar in meaning to “is useless” in 4:14b. In some languages the same word or phrase would be used in both places for the correct meaning.

the promise: The word promise here refers to what God actually promised to Abraham and his offspring, that Abraham would be heir of the world (4:13a). Make sure that is clear in your translation. For example:

that promise
-or-
the promise ⌊that God actually made⌋

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