We bought one a few summers ago from Millets, but make sure you get one that you can stand up in, some of them are a bit low and its means you have to crawl around all the time...our one has bedrooms off the main living area.
If you're still looking, my first bit of advise would be, get a 6 man tent if there are four of you otherwise you'll find you're all on top of each other if it rains.
Secondly, depends on what style of tent, SIG? Pitch inner first or all as one? Dome or tunnel?
The 'rain speed resistance' always seems a little less than the numbers suggest, so best to get one a little MORE than adequate for what you might need. You also might have a later use for the thing, so may as well get one that'll last and be good for other trips.