Right now, the requirement is 600 million Cr in scan profits, although a more precise one would be somewhere around 620-640 million: I just opted to go with 600 because it's easier to remember, and the difference isn't all that large anyway. In Chapter Three, one can scan an already-discovered ELW for around 650k Cr (depending on mass, it could go as high as 700k), or with the first discovery bonus of +50%, 975k Cr - let's just go with one million there. While WWTCs can go even a bit higher than that, for the sake of simplicity, I'll just unify both kinds as a "rare body scan", with 650k Cr discovered, and 1M Cr undiscovered.
With that in mind, current Elite Plus status would be 924 discovered rare body scans, or 600 undiscovered scans. Let's see how that changes in Chapter Four, assuming my beta values will be correct for launch.
For the first time around, the mechanics of scanning bodies will change, so it's not just a numerical value change. FSS scanning replaces D-scanner scanning, and mapping via the DSS is added. Since the FSS takes less time, payouts for scans were generally decreased. Thankfully, the time it takes to fly to a body and probe it is roughly the same as before, so I won't consider the time required beyond this. The bigger change instead is the first discovery bonus: silently, Frontier raised that from the original +50% to +220%. Whoa. I'm not sure why exactly +220%, and not +200%, but perhaps it's a matter of a combined first discovery and first mapping bonus. I couldn't separate the two yet, but for our purposes here, it doesn't matter much: we can just safely assume that our hypothetical explorer will either come across an ELW that's either already double-tagged, or not discovered at all.
There's also the matter of a +25% efficiency bonus. For planets the size of Earth-likes, that's generally 6 probes, so if you cover 90% of the planet with six probes or less, you get a 25% payout bonus. With an engineered DSS, hitting that target is quite easy and should quickly become routine, so I'll just roll that in. (I could assume that a Commander doesn't bring an engineered DSS and might miss the target from time to time, but that would be similar to assuming that a Commander didn't bring a DSS in Chapter Three and earlier.)
So, with all that, what values are we at? For a similar-sized ELW to the 650k Cr discovered / 1M Cr undiscovered mark, it'll be 1.35M Cr discovered (+ efficiency) / 4.3M Cr undiscovered (+ efficiency). Roughly doubled if discovered, and over four times as much if undiscovered. That presents a dilemma: which to go with? This was why I did a quick recap at the start: the meaning of Elite Plus was to maintain parity since the beginning of exploration. Now, in the beginning, we didn't have the journal, nor crowdsourced lists of valuable planets. If we are to assume that a majority of the rare bodies a Commander finds now are already discovered, then she pretty much has to be using some external tools to do that. Since said tools weren't available early on, let's go with the undiscovered values instead. That's much more like exploration than just visiting known sites is, too.
Let's see then. 600 undiscovered scans in Chapter Four would be 2.58 billion Cr - 2,580,000,000 Cr, just to make sure. Again in the interest of simplicity, let's round that down a bit, to 2.5 billion. (I was actually tempted to round it up because WWTC might turn out to pay considerably more, but I decided not to assume that the majority will be such.)
Update: it looks like I was wrong, and the 4.3 million Cr payout I used was only possible due to a bug. Recalculating with a median 3.25 million per rare undiscovered find, the requirement is rounded to 2,000,000,000 Cr instead.