Dude, you need to find a new job.... and fast.  Seems like your company does not value you enough to find a way to keep you beyond your 6 years of H1B allowed time.  Why not look for another job that will process GC for you and use the 4 years at this company as your experience?  Their insistance that they will be easily able to find a US candidate to do your job is lame.  Pissed me off.  Companies usually find a way to make your job unique if they truly value you as a precious resource and do right by you.