You're comparing characters with integers, so currently you're comparing them with the control characters U+0003 and U+0004 - you want to compare them with characters representing the digits:
if (zipCode.charAt(0) <= '3')
...
if (zipCode.charAt(0) >= '4')
if (zipCode.charAt(0) <= '6')
(You might consider using a switch statement too...)
I've no idea whether this is correct in terms of what zip codes mean, but that's the immediate problem with your code.