This is probably all resolved by now, but I'll add my two cents.
What do you mean exactly by "listed as"? Did they really misrepresent? Here's the thing- there are tons of listings on our local MLS that list all the bedrooms, conforming or not, in the main part of the listing. Then in the room details they mention "nonconforming", or perhaps in the description/notes. This may sound on the surface as if they are being shifty, but I don't think that's the case. I mean, if you'd seen this house listed as a 2 or 3 bedroom would you have gone to look at it? It probably wouldn't have even turned up in your mls search.
You can always ask for concessions of course, and you may as well try it.
Sounds like there is only one basement bedroom? Egress windows are only required in bedrooms, not living/family room areas.
You can of course rent it as a non-conforming 4th bdrm, or as a 3 bdrm with office, whatever. (believe MJ Barret's advice above would apply in CA but not here- these laws are state specific.)
But you will increase the value with an egress window plus you'll sleep better at night having done it.