Super Mario World is Super Mario Bros. 4. That much is indisputable. The unanswered questions are:
- How do you add it all up, considering there are two different Super Mario Bros. 2s? (TLL and USA)
- Is Yoshi's Island Super Mario Bros. 5, considering it is Super Mario World 2?
- If so, is Yoshi's Story (which was originally going to be titled Yoshi's Island 64) Super Mario Bros. 6, and Yoshi's Island DS (which was originally going to be titled Yoshi's Island 2) Super Mario Bros. 7 (or 6, if you don't count Yoshi's Story)?
- If you do count Yoshi's Story, then is Yoshi Topsy-Turvy (aka Yoshi's Universal Gravitation) Super Mario Bros. 8?
- And of course, let's not forget the Game & Watch Super Mario Bros, and the PC-88 title Super Mario Bros. Special.
If you then decide that Super Mario Land deserves to be included at least as much as Yoshi's Story does (which it does -- especially SML2, an underappreciated platforming classic that still feels fresh today), then before you can even decide what numbers of SMBs they are, you have to figure out their own numbering.
- Super Mario Land 3 is Wario Land. Does this mean that Wario Land 2 is Super Mario Land 4 (by the same logic that counted Yoshi's Island as an SMB)?
- If so, is Wario: Master of Disguise counted as Wario Land 5 (or 6, if you count Wario World as 5) and therefore Super Mario Land 7 (or 8)? (Wario Land Shake would be SML 7, 8, or 9, depending on how you counted MoD and WW).
Adding in New Super Mario Bros. and New Super Mario Bros. Wii, the upper limit on the number of SMBs is at least 22, with the Game & Watch and PC-88 games included and counting both SMB2s (and 29 if you also count Donkey Kong Country 1-4 and Donkey Kong Land 1-3). And then there's more debatable cases like Super Princess Peach, Super Paper Mario, Donkey Kong Jungle Beat, and so on.
I'm being overly hyperbolic, of course, but my point is that the numbering system is complicated. We're definitely at least up to SMB6 already.
Incidentally, I think what most people really mean when they say they want a "Super Mario Bros. 4" is that they want a "Super Mario Bros. III-2".