I haven't used fontswap, but when I had my 3GS, I manually went into /system/library/fonts/cache/ and then switched out the Helvetica (system font) with the font of my choice. However, that was using a .ttf file, and I went in and tried to do it again, but it looks like the iOS4 on this iP4 is using a .ttc, which is an entire collection of fonts for Helvetica, versus one font for bold, italic, thin, etc. I'm guessing maybe that's why fontswap is not working.