"R4" is not a generic term for flash carts, but many clones call themselves some R4 variant because they know that the average person doesn't know any better.
You have three choices for carts that have proper support/updates.
The original R4.
NOT "R4-III Upgrade", NOT "R4-SDHC", NOT "R4i NDSLL", NOT "R4 Red", NOT "R4 King"...
NOT ANYTHING ELSE WITH "R4" IN THE NAME! JUST "R4 Revolution For DS".
- Price: ~$6
- Rank: The cheap card for somebody that just wants to play games on a DS/Lite.
Acekard 2i
- Price: ~$16
- Rank: Mid-range card for people that want to play games on a DSi/XL with some extra features.
Supercard DSTwo
- Price: ~$35
- Rank: Pretty much the ultimate card right now.