The “3DS” in the name of numerous (cheap) flashcarts is a little misleading – they are still flashcarts for DS games. When the DSi was introduced Nintendo started fighting against Slot-1 carts. All DS games that existed at that point were put on a whitelist – newer DS games contain a signature.
If you insert a DS cartridge into a DSi or 2/3DS, the console looks if it is on the whitelist – or has a valid signature. If not, the “game” is ignored.
Newer flashcarts pretend to be a retail game (on the whitelist). Therefore the flashcart must contain at least parts of the ROM of the game it pretends to be. With this trick flashcarts can be used on a DSi and… the 2/3DS systems. (I’m omitting the cat and mouse game Nintendo played for a while)
Your R4 3DS SDHC Gold Pro also pretends to be a DS game. If you start the “game” the 3DS reboots to DS-mode (TWLFIRM, another operating system) and at this point essentially becomes an old DS (reduced resolution, no 3D, not using the ARM11 main CPU, no access to the NAND and the SD inside the console, 4MB(or16MB) RAM only) and is not able to start 3DS software at this point.
Flashcarts for the 3DS mode exist:
- Gateway (and clones) → Uses also a custom firmware, outdated and dangerous nowadays. Does not work on current firmware.
- Stargate → As far as I know from the same team as Gateway,
also a CFW. thanks @maorninja for the correction. I should have read more carefully instead of just looking for some buzzwords in the manual:
- 3DS functions like Sky3DS+, same limitations as below.
- Can act as normal DS flashcart as well
- Can act as ntrboot flashcart for installing CFW.
- → But, without searching for sources now: I've read here on GBAtemp that there are some problems with Stargate and installed B9S/Luma.
- Sky3DS(+) → This needs no modification on the 3DS. But is expensive and has many limitations:
- Only plays unmodified, encrypted ROMs of the same region as the console; this means:
- No Eshop only games.
- No romhacks.
- It basically mimics the legit 3DS game of the currently loaded ROM.
- No additional functions like the R4 offers for DS games.
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The decision about homebrew and CFW is up to you – of course – but everybody here can tell you: It is safe unless you do stupid things
on purpose and ignore big red warnings that pop up before doing these stupid things. And even then: With the R4 you have, you can always recover from any software damage (keeping a NAND backup is a must).
Reading the guide surely does not do any harm. The guide is long, but only because it is a step-by-step guide.