Signed up for a talk, not enough material
Twenty minutes is not long for a talk.  Moreover, most students drastically overestimate how much material you can put in a short talk.  Giving a short introduction to the field and how it came about will be more than adequate to fill that amount of time, and would be a perfectly reasonable topic for an incoming PhD student with only four months in the program.  Time permitting, it might also be nice to mention your research question and the general avenues of inquiry you are going to use to solve it.  You should also leave a few minutes for questions and comments, particularly since this will give researchers in the audience an opportunity to offer suggestions on your research idea.
So no --- you didn't make a mistake signing up for this talk.  Don't bother trying to cram out new research results for it --- just stick with your plan to give a short introduction to your field and a brief outline of your research question.  That should be enough to hold people's interest and solicit some questions, and the twenty minutes will go by in a flash.  I doubt a short talk like this is really going to help your CV all that much, but it will be good practice for giving conference talks, and if you are lucky, it might precipitate a longer conversation on your research idea with one or more of the researchers in the audience.
(And as Buffy points out in the comments below, no one will be upset with you if you don't use all your time --- running over time is bad, but having time to spare is fine.)