I personally wouldn't worry about their weight rating if you'll just be cruising around. Just get out there and ride!
True. But do be careful where you ride. If you ride off a curb or over a big pot hole you could easily break some spokes or cause some frame damage. This advice applies to people of all weights but extra so when your nearly 400 pounds.
Some alternative advice, if your actively trying to lose weight, why not try to get down some pounds and reward yourself with a really nice bike. Your options will be much larger as many bikes are rated for 250-280 pounds.