This achievement is missable!
The way it works: Each Chest, Field Icon item (!), and Reward Item count as one point. Each Chocograph Chest, Crack, or Bubbles count as 2 point. Starting at Rank H, you have to progress through ranks G, F, E, D, C, B, A, finally to S.
Points required: Rank S requires roughly 3/4 of all points available over the course of the game, for me it popped at 373. It shouldn't be very difficult if you're diligent in searching each area but apparently I plowed past a ton of items and came up short on my first playthrough.
You can check your current rank in two places: The woman at the bottom of the Inn in Treno, or the four armed man on the right side in Daguerro.
To get the Rank S Medal for the achievement, speak to the four armed man in Daguerro once you reach Rank S.
In total, there are 135 chest/field/reward locations available on disc 4 along with the 32 Chocobo related items. That makes for only a total of 199 points available at end game, meaning you have to get 141 along with way to that point to avoid missing it. The total game breaks down as such:
143 Field Items
89 Rewards (not every one can be done in a single playthrough though)
25 Chocograph Chests
5 Chocobo Bubble Locations
3 Chocobo Crack Locations
Making a total of 477 available points where ~370 is all that's required.
Huge props to Atomos199 for making this guide that I'm using in my second playthrough to ensure I don't miss it again! It contains a full walkthrough for the game as well as a complete area item list, including indicators for every item still available in disc 4:https://gamefaqs.gamespot.com/ps/197338-final-fantasy-ix/faq...
For all the Chocobo information, I found this site pretty helpful:https://jegged.com/Games/Final-Fantasy-IX/Side-Quests/Chocob...
And finally it is possible to get all of the required points as soon as you get to Daguerro for the first time after getting access to the Hilda Garde III airship, but it's attainable anytime after that.
EDIT: I finally wrapped this up using a playthrough of just full boosters and counted every item along the way (I think). From the list I linked up to earlier, I was C Rank when I got to Daguerro for the first time and here's how many items I had at each check:
318 = C Rank
339 = B Rank
353 = A Rank
357 = A Rank
374 = S Rank
I did absolutely zero Chocobo work, zero frog catching, and zero jump roping (I was also going for Excalibur II, so was gonna do that stuff after getting it if necessary) and was able to get it as soon as I returned from Pandemonium having picked up nearly every item available to that point. For posterity, here's my list ("X" = Item I got, "-" = Item I missed, "*"=Item I didn't get but still available on disc 4):