Dear 1st timer,
First of all, OH YOU'RE GONNA HAVE SOME FUUUNNN!!!
But in a word, yes! You can do all 4 parks in 6 days! We only take 6 day stints at WDW, and we hit all 4 parks EASY in those days.
But we are Park Commandos. We haven't been in our hotel room during daylight hours. All waking hours are spent within parks or eating at some restaurant. We don't take time out to go swimming, or relax and watch TV. We have a pool that we pay much HOA dues for, so we take a picture of the pool and keep it moving to the buses!
Oh, and we've been going to WDW for 3 years straight and we haven't seen what Downtown Disney looks like, either. Why? Because Atlanta have great malls.
So I go to WDW to ride. And watch the shows. And take pictures. And eat. And play until I'm so physically tired, I pass out on the bed.
But once daylight peek through the window of the hotel? We're gone to do it all over again!!!
(By the way, I'm like God in this sense- on the 7th day, I rest!