The guidance cantrip says the following (PHB, pg. 248):
You touch one willing creature. Once before the spell ends, the target can roll a d4 and add the number rolled to one ability check of its choice. It can roll the die before or after making the ability check. The spell then ends.
The Combining Magical Effects section says the following (PHB, pg. 205):
The effects of different spells add together while the durations of those spells overlap. The effects of the same spell cast multiple times don't combine, however. Instead, the most potent effect–such as the highest bonus–from those castings applies while their durations overlap, or the most recent effect applies if the castings are equally potent and their durations overlap.
If there are 3 PCs (Alice the Cleric, Bob the Druid and Charlie the Rogue), and two of them know guidance, can they both (Alice and Bob) cast it on another PC (Charlie, who is about to make an ability check) such that Charlie can use the better of the two d4 rolls, effectively rolling the d4 "with advantage"?
Obviously the d4s don't combine; rather, if the d4 from Alice's guidance rolled a 1 and the d4 from Bob's guidance rolled a 4, then Charlie gets to add the 4 instead of the 1 to his ability check?
No, they don't
In the same section (PHB, pg. 205) that you've quoted, it also gives the following example:
This effect is similar to the Guidance spell, so Charlie will only get one extra d4.
The wording most potent effect in this scenario does not refer to a roll result, but rather to the number of dice, or a fixed effect. For example, if you cast a spell that gives a 2d4 bonus, it will overrule a spell that grants a 1d4 bonus.
Fore more information on overlapping spells and which effect is more potent, check out this question