With four people - you will probably want to take a cab. I did the Mears shuttle between WDW and US/IOA and it did go smoothly for me - but once you hit four people - I think the cab is about the same cost. Occasionally you can snag a cheap rental car - but that takes planning or luck!
Yes - you can do IOA in one day.
1) Buy tickets online before you go. It's cheaper and their ticket lines were long the day I was there.
2) Plan PLENTY of time to get there, walk from the taxi/shuttle drop off, get through bag check, and be there when it opens and follow the crowd to WWoHP. You probably need to leave the WDW area at least 1.5 hours before park opening time if going by cab. For the shuttle - at least two hours as there may be another stop.
3) If you are there FIRST THING - you will get into WWoHP and won't have to get a timed ticket to come back later in the day. Now - once you leave that section of the park - you may have to deal with timed entry to get back in later in the day - depending on the crowd and time of day.
4) Do everything you want in WWoHP before you leave that section. Then - enjoy the rest of the park. If you plan to do alot of the other rides in IOA outside of WWoHP section, it might benefit you to get an express pass. If you aren't big ride people, you could probably skip the express pass. If you really want to hit all the big thrill rides - this will make your life easier. It is definitely more money, though
5) Unless you have teenagers that eat a ton, I wouldn't suggest the meal deal. Can't use it in the HP section.
It does feel pricey because you'll be paying the one day ticket price. Disney is high, too, for just one day. If you really plan to make the most of it, I think you will get your money's worth.
The one mistake I made was not really understanding the rides and shows at Universal/IOA before going. I didn't use my first day at the park as wisely as I should have. I was exhausted/done for the day - but in many ways - I just really oriented myself to the park on my first day. So - spend alot of time on their website, take notes or print things out. Otherwise - you can waste alot of time doing nothing. I learned all about WWoHP in advance - but not about the rest of the park. That was a mistake. I should have spent more time figuring out what I was going to do AFTER my HP fix. IOA is a nice park - good theming, variety of things to do, be prepared to enjoy the rest of it, too. I thought I wouldn't be interested, but ended up wishing I'd knew more about the rest before going.
I love HP and for HP fans - the price and crowds are worth it. You can spend 3-4 hours in the WWoHP easily if you are a fan (some can spend longer) and I definitely recommend eating at Three Broomsticks to say you did (and drinking Butterbeer). Bring a pen and buy postcards to mail from Owl Post with a Hogsmeade postal code!
