I think a used S&W M&P pistol would be a better deal than a new sub $400 gun.
That said there are a lot of good, new guns you can get for $400 and under. Look at the Taurus line, Kahr cw line, S&W SD, Ruger (I even see SR9s for $400 sometimes), just to name a few. The Sig 2022 for $400 is also a great buy. I had to keep myself from getting one the last time they were at Academy for $399. That's a great gun for that price.
That said there are a lot of good, new guns you can get for $400 and under. Look at the Taurus line, Kahr cw line, S&W SD, Ruger (I even see SR9s for $400 sometimes), just to name a few. The Sig 2022 for $400 is also a great buy. I had to keep myself from getting one the last time they were at Academy for $399. That's a great gun for that price.