My wifes due date is June 06' and my sister in Hawaii is buying us tix to go there for Dec.06
Is it ok for our baby to be in a plane that early? flight is about 4hrs from here. (california)
My sister says its ok, but Im already worried.

Check with your peditrician for proper advice. He/she will know your baby and what is best. I have to admit, the air on airplanes are not the best even for an adult to breathe.

My DD took her 1st trip at 3 months, on a plane. Ask you Dr. It is perfectly safe for them too fly, as long as they are healthy and have no other issues.

When my son was born we lived in a different state from the rest of the family and when he was a month old we had a family emergency at our family home so we flew twice once at 4wks then 6 wks, again we flew back home for a visit under less stressful circumstances when he was 4.5 mos and back to were we were living a month later. Then we moved back to home permanently when he was 8 mos old. His pediatrician said it was fine to fly when young as long as not sick. Best is to have flights straight through with no lay over and use a pacifier, bottle or if breastfeeding during take off and landing for the pressure change. My son did very well on flights and actually usually slept through the whole thing as we did later flights so he wouldn't be bored.

Make sure to ask for a bulkhead seat (these are the seat quite in the front, with no-one in front of you. It means the flight attendants will be able to attach a table with a small bed or carseat in front of you, so you don't have to hold your baby for 4 hours straight. If you are lucky enough to have a seat for your child make sure the carseat you are taking is airline approved (it will have a sticker on it that says so, but better yet is to check with the airline.
I have taken all my kids all over the worls from 6 weeks onwards. Flights of 3 hours and flight of 17 hours onwards...I find traveling with small babes real easy: they sleep most of the way (the noise lulls them to sleep). Do feed them when taking of and landing so the change in airpressure doesn't hurt their ears. Also: when they say they will take off it usually last another 10 minutes before you really take off so don't put your baby on the bottle or breast before you are actually speeding up on the runway otherwise you will find yourself with a completely full child even before it is time to protect those ears!

have fun in Hawaii!