For an iPhone app though? Meant to be installed over the air? That's incredibly poorly thought out.
Better would be to allow you to download which maps you need as you need them, to shrink the initial installer down. Better than that would be having the maps be online accessible, and are immediately cached to the device after first use. Best is probably somewhere between those two, or something along those lines, but I can't imagine who packaged up a 4.2Gb iphone app and didn't ask any questions about that.
It's probably more robust to let iTunes download the whole binary, instead of have the user attempt to download specific maps over WiFi (the device may go to sleep, run out of battery, etc).
Funny how I tried to download iOS 4.3 from iTunes for my new iPhone 4 yesterday and the computer went to sleep while downloading it, which is around 700Mb. When I noticed I woke it up and clicked on resume, bang, went from 400Mb to "zero Kb" right away.
And, uh, it happened twice... Finally, at my third attempt I was able to download the whole thing after deactivating automatically sleep mode. As soon as it finished, an error was shown and so the file was lost again.
P.S. Mapping software is by nature huge. It typically contains, you know, maps. Of the whole country.