With many builders have extreme layoffs, your best bet would be to contact the builder at their main office. Fair warning though, slabs with cracks (no matter where they are) are very rarely covered by any builder, especially after 4 years. Add in the state of the economy and they're going to be even less eager to respond. Also, my guess would be that the slab is not covered under any structural warranty... much like your driveway, it is nothing but concrete on subbase and has no factor in the structural integrity of your home (of course there are instances that it can be a symptom of something else being wrong, but nothing more). Of course I haven't read your warranty but I'd be surprised if it was covered... always worth a shot though! Good luck!