4 hour lay-over mean you probably have 2 hours to spare in between flights once you get off and need to get back on. You will only be able to make a quick trip to downtown and back.
You're best bet is to check out the USS Arizona - Pearl Harbor - just a quick drive away or bus ride (#62) away.
A longer trip to Waikiki or Diamond Head will be a tight squeeze, and you'll probably only be able to be there for 15 minutes -- but you can take a quick pic and say you've been there. almost 9 years ago