Christmas is an odd time of year. It puts many people into awkward positions of obligation and quests for "the ultimate gift" upon which the givers base their relationship with the recipient based on their reaction to the gift. It is not WHAT you give but the spirit in which it is given (as well as received).
It seems to me she gave you the chance to put in your request for a Christmas present. I understand you were trying to be considerate of her financial position, HOWEVER, when asked you should have given her an answer. Even if it was something as simple as a plate of your favorite cookies. It does seem a little odd that after 4 years she doesn't have a bead on what you like but maybe that should tell you something about how observant she is and where you land on her list of priorities. I'm not trying to sow dissent here but it sounds like you rank second to the rest of her family. Don't get me wrong, family is important--especially one's children--but it seems after four years you should be a bit more deserving of a hurried phone call asking what you'd like for Christmas while she's "out shopping for everyone else."
It seems your GF is somewhat hung up on the monetary value/quantity of gifts, not their quality. Don't get me wrong, buying her a 42" TV was nice but did she really NEED that? Does she look at it and think of you fondly when she's watching "Dancing with the Stars" or PBS? The gifts that I most cherish are the ones that people made for me. I have so few of them but knowing that the person took the time to fashion them by hand with me in mind makes them so much more valuable than an iPod or flatscreen TV or gift card.
That being said, I would argue AGAINST the revenge tactic. You could always say,"That TV wasn't cheap so you b'day gift isn't going to be overly spectacular" plus you COULD argue that the TV wasn't for her but mostly for her sons. BUT, That kind of smacks of laying the guilt card on her. Granted, financial reality is what it is: if you want something expensive now, you'll have to sacrifice something in the future to get it. That's a fairly simple equation and it would be very telling how she reacts to this news if this is, in fact, the ploy you use. If she becomes indignant and impatient with the fact that you spent money on the TV for Xmas and don't have anything for HER birthday, that should tell you something (and it's not good.) On the other hand, if she tells you, "that's OK. I'd be happy just with an evening home alone with you" you've got a good soul there; hang on to it.
Is this the first time this has happened or does this seem to be a recurring theme in your relationship? I understand that children always come first but it seems to me you've put in enough time and effort to be considered "family" rather than second fiddle. Obviously, I do not know the finer points and history of your relationship and if this is status quo and you're happy with it, who am I to tell you to do anything differently? But it sounds to me like you expect a little bit better. Does she? Have there been instances perhaps where she expected something of you but you inadvertently ignored the request? Indifference can be a two-way street. I'm not judging; I'm just asking.
"Relationship" means having to forgive the other person when you're hurt and apologizing when you've hurt your partner. That's not always easy for humans but some do it better than others. Relationships ARE work. They EVOLVE. They require constant attention and vigilance and communication. You could simply outright tell her that you're feeling a little disappointed with the inequity of the situation. And I'm not talking financial inequity (though if you want to play that game, you're probably in for a long, drawn-out argument) but emotional inequity. The fact that she told you she "had no money for Christmas" is immaterial. It's the fact that after taking care of EVERYONE ELSE in her life, THAT'S when you popped up on her priority list. Yes, children, parents and siblings are important. But don't you figure into that as well? Or are you "just" a boyfriend?
Perhaps the best way to deal with this is let it pass, as difficult as it may be. When her birthday rolls around, DO something for her form your heart. Don't GET her anything; DO something for her. Take her out for a nice evening. Fly in an old friend whom she hasn't seen for years for a weekend. Again, it's not the COST of the gift, but the MEANINGFULNESS of it. If you're meant to be, she'll get that. And she'll be grateful.