Right but the sentence in question is talking about the bona fides of four men, hence the need for the plural of “to be”.
The Oxford dictionary cites the formal (correct) usage as singular and the informal (incorrect) usage as plural. It doesn’t matter that there are multiple candidates since the reference is to the bona fides of each individual candidate, not the sum total of all bona fides of all candidates.
You wouldn’t say “the intellect of the 4 candidates are well-regarded”, but you instead would say “the intellect of the 4 candidates is well-regarded”.