got my wife a 9mm hi-point. very inexpensive, excellet warrenty. and a trouble free gun. shot about 150 rounds so far with NO problems. would not hesitate to buy another

I would get no less than a 9 but 40 if she can handle it. maybe find somewhere that u can try em out. if she feels comfortable with the 40 then get it. since she wants one then she will probably handle the 40 better than if you tried to get her to shoot it and she didnt want to. once you find out what she can handle, take her and let her feel a few and pick out her own gun, after all she's the one shooting it. but also you make sure she gets a good quality one. try to steer her toward glocks. easy to use and not a bunch of moving parts, just rack it back and let er fly. also very reliable, if its to protect her life you certainly dont want a cheap gun thats gonna jam after one shot.

no doubt
 

you said it right there, I got one loaded up under the bed with a surefire foreend molded light on it. anyone breaks in, lord help em is all I can say. I also got a glock23 in the drawer beside me.

the m4 would be a great home defense gun for a conscious and safe firearms user, however you dont really want a scared wife with limited gun experience spraying high velocity rounds through your house cuz that can go through several walls including exterior which can be very dangerous especially with children around.