I have read one other post about this, the poster said FontSwap changed his font on iPhone 4 but he could not get it to change it back to default. So, he had to restore.
As for me, I will not use it until I read about more successful uses of it.
Unfortunately, Fontswap is not yet compatible with the iPhone 4. I think I read somewhere that they are coming out with an update, but haven't seen anything new. Check out the following two links, I was able to SSH the fonts that I wanted over to my phone.