Bike riding has a lot of benefits like being easy on the joints but it is not very good for weight loss, if you do bike riding make sure it is "in addition to" and not "instead of" whatever you are already doing. I have a walmart bike and weigh 300. I had to upgrade the tires and tubes already which was 60 bucks, plus add seat padding for 20, get a bunch of tools and crap to constantly tighten the spokes. About 120 bucks so far, aside from the bike itself.
At this point I think I might just get a 500 dollar bike from mcghies and save myself all the headaches I have gotten into, and this is 'just' at 300 pounds, and only riding a modest 4 miles a day. At 400 pounds you may have even more trouble, though the 4 inch tires should be a giant help.