From the Wikipedia-page:
Most flash devices are designed for about 100,000 - 1,000,000 write
operations (or "write cycles").
As you can read, the typical NAND flash has 100k+ write cycles, so I won't worry about that. Besides, I'm sure Apple has figured out some kind of system to increase this number by making iOS "drive-smart", meaning they've optimized the way iOS is reading/writing data to the flash in order to extend his life-cycle.
Like XAleXOwnZX said in his comment, the NAND flash memory will survive most iPhone life spans.
And another fact, it will take you about 140 years to wear out the same flash location. Most of the time, the data stays in the cache rather then it will be written on the flash memory.
I hope this answer comforts your concerns :)
EDIT: I've found what kind of flash is used in an iPhone 4. It's Samsung K9PFG08 flash memory and this blogpost tells you more about the typical reading and writing data and kind of confirms my findings above.