The next of the Four Reliances is either the third or the fourth, depending on the source, and it's also the one that seems hardest to translate. Sometimes it is rendered "Rely on explicit meanings, not implicit meanings." Sometimes it is "Rely on final meanings, not provisional meanings." I've also seen "Rely on sutras that lead to enlightenment, not on sutras that don't lead to enlightenment."
The confusion seems to come from the words nitartha and neyartha. I take it that the original Sanskrit says to rely on nitartha and not neyartha. According to an online Sanskrit-to-English dictionary, nitartha means "of plain or clear meaning" and neyartha means "having a sense that can only be guessed."
Some interpret this to mean that we are to rely on texts that are unambiguous and not on those subject to various interpretations. That would mean we can't rely on this Reliance, since it's interpreted every which way. Truly, there is nothing expressed with words that isn't subject to interpretation, including yes and no. But since we've just been told in the second Reliance to rely on the meaning and not the words, let's assume we're not talking about words here.
Tibetan Buddhist commentaries on the Reliances seem to lean more in the direction of provisional/final teachings rather than implicit/explicit teachings. This works for me, and if you interpret the Reliance this way, it makes a better complement to "Rely on the meaning, not the words."
In Mahayana Buddhism, the teachings of the historical Buddha as recorded in the Tripitika are considered to be provisional; the first turning of the dharma wheel. Many Mahayana teachers urge students to always consider their current understanding to be provisional or imperfect in order to remain open to deeper understanding. This is relying on the final meaning, not the provisional meaning.
I also found a text not directly tied to the Reliances that turns the explicit/implicit advice on its head. Alexander Berzin, a scholar of Tibetan Buddhism, explained that a "vajra expression" can have two meanings. The explicit/nitartha meaning is the "face value" meaning, or what the expression appears to say. The implicit/neyartha meaning of the same expression is the "meaning to which one needs to be led and which one must come to ascertain."
As I read this, another way to say this is that nitartha is the literal meaning of an expression and neyartha is the wisdom to which the expression is pointing. Or, you could say the first meaning is a metaphor for the second or somehow presents the second.
I've seen that it can be painfully difficult for some people to even grasp that a text may be saying something other than what the words literally mean. Some historians say that the way we humans define "truth" changed during the Scientific Age that began in the 16th century. The notion of "truth" as something actual is relatively recent. This shift in the way we sort information in our heads may make it harder for us to perceive the kind of truth presented in ancient texts.
I like this Reliance as number three instead of number four in the list because it sets up a kind of tension that is resolved by the next Reliance, wisdom.