Christmas Rushed: Battle Network has always released its Japanese installments in early December (with an English release in Summer of the following year), but Battle Network 4 got a vastly shortened development time, as the first few months of the year it was released (2003) were also spent developing and releasing Battle Network 3 Black. BN4 was also a vastly more ambitious project, being One Game for the Price of Two whose versions were released simultaneously with large quantities of new and independent content (i.e. unique characters, chapters, and cutscenes), all while Capcom was going all-in on merchandise like Patch Cards and toy BattleChips that the games needed to be compatible with. If that wasn't enough, ROM space concerns and the general graphical trend of late-era GBA games meant that everything except the battle sprites needed to be redrawn to be smaller and brighter. The New Game Plus system is an obvious attempt to make the best of what little single-player content they were able to complete under these conditions.
Dummied Out: The Patch Cards obtained via the Nintendo e-Reader are still in the international versions of the game and fully translated. However, since the e-Reader received very limited international support and the cards were never released outside of Japan, there's no way to obtain them without hacking. The Legacy Collection port officially implements them into the game worldwide.
Franchise Killer: While Battle Network 4 was very successful by the series' standard (it was not only the best-selling Battle Network game, but also the third best-selling Mega Man game of all-time after Mega Man 2 and Mega Man 11), such a poorly made game released at the height of the series' popularity dealt an irreparable blow to its reputation. While the series did get to see its way to completion, the tarnished reputation led to 5 and 6 not selling well despite being much better received (even when taking 6's release in 2006 on a platform most devs had declared themselves "done" with), and the series has remained dormant outside of some re-releases since 6. It's notable that the complete collection, when many companies including Capcom were releasing remastered compilations before it, came out in 2023 when the rest of the Mega Man games had released around 5 years earlier.
What Could Have Been: Double Souls based on Bass and Duo were planned, but had to be cut due to memory limitations. Bass Soul would end up being reworked into Bass Cross in Battle Network 5.
Other Trivia:
Blue Moon has very major emulation issues that make can make it borderline unplayable due to game-breaking bugs, specifically in the WoodMan scenario. These problems actually affect the game's playability on the original model of the Nintendo DS (making it one of the few, if only, GBA games with any sort of DS compatibility issues), which Nintendo was forced to acknowledge on their website.
