^ This is definitely the best of all, especially if you're worried about theft, weather damage, or vandalism. It's also the most expensive, but maybe not if you're in a really high theft/bad weather area.

Other things to consider:

Top racks have a few problems. One is forgetting that the bikes are up there and driving them into a low entrance way. Another is bug guts getting on your bikes if you take them on the highway (trailer carriers have this problem too but to a lesser extent). Finally, you have to be careful with how you pack things--putting the bikes you need most in the middle will be a pain, as you have to remove the bike on the side to get to the ones at the center easily. Some rear racks also suffer from this problem, but it's usually easier to get bikes on/off them.

I'm guessing you'll have 2 adults and 2 kids in the car, so you won't be able to lower the seat back of the rear sears. But, if the kids' bikes are small enough, try and fit them in the trunk. Or, if you have a really nice bike (CF frame), put that one in the trunk and leave the other, less expensive bikes outside.