Don't let them give you any guff about the tank. Besides the problems of you running out early, a leaky oxygen tank is a rather large safety hazard.

Though I can see another possibility... It may be good for 4 hours at full pressure, but your home setup isn't filling it that full.

Or they gave you the specs for tank X, but supplied you with tank Y. (unless the specs were packed with the tank, I'd suspect that)