I don't think there really is such as thing as a "beginner bass" in terms of anything musical. You can learn to play on a $2000 Rickenbacker just as easily as you can learn to play on a $200 Squier. There are no training wheels.
I think what people usually mean is a cheap but adequate bass that they can start learning on without spending too much money. It really comes down to budget.
Is your $400 for the bass alone or for your whole setup? Are you planning on getting in with a band as soon as possible or is this just to play at home for the time being? If you have your eye on a band, you seriously need to budget for an amp that can keep up with a drummer, which means something upwards of 100 watts. If you're just playing at home, pick up a used bass practice amp off Craiglist for $25-50 and spend the rest on your bass.
Yamaha is a good reliable brand, and the RBX170 is their budget model. If you have $300-400 for a bass alone, you might look up the line a bit at their BB series; musiciansfriend has been selling the BB424 and 425 for about $300 lately, which is a great steal. The BBs are great basses, although they have these wide bodies that I don't find too comfortable to play sitting down (you kind of have to wrap your arm around them). Very good feel standing up.
Squier has the Vintage Modified and Classic Vibe series in that price bracket, which I've never heard anyone say anything bad about. SX (at rondomusic.com) had super-cheap basses (starting from $110) that are usually still pretty good, though taking it into your local guitar store for a setup is recommended.
And the usual pieces of advice - try before you buy, at least to see if the bass feels comfortable (e.g. some have chunky necks, some have thin necks, and different people have their preferences); and remember that buying used usually gets you more bang for your buck as long as you're not buying someone else's problem (if you have any bass-playing friends, take them with you to make sure the neck is straight etc.).
