Even at $400, you can afford to refine your search for the best possible fit. Metal or polymer? Compact or full-framed? And $400 can get you into an excellent/used formerly $600 gun. If you go into a gun store with a $400 budget, a lot of places will steer you toward Taurus (because seemingly EVERYONE is overstocked on them). Some are fine, but I'd be a bit apprehensive about running with the Bull.
A few suggestions:
S&W SD40 (the new & improved Sigma?)
Used polymers (XD and Glock are easy to find around $400)
Used S&W Model 4006 variants
The above can all be found for around $400. And if you keep your eyes peeled on a local internet trading forum, there are occasionally great used deals. I've seen used Sig P226/229 and XDm40 go for $400-500 on occasion.
SI VIS PACEM, PARA BELLUM
People often ask me, "How many ARs do you have?". Well, that seems to be in a constant state of flux. An honest and concise answer might be something like "six and five-eighths". And to the AR enthusiasts among us, that will make perfect sense.