Forty hz is an awfully low upper limit for an intentional* design. You would have too much energy concentrated down in the sub-sonic area...it would be like tuning a vented box to 15hz. Having a big gap in the midbass from say 60-100hz is going to sound horrible.
*It's actually going to be hard to limit it that low without some exotic multi-order design unless your particular transfer function coincides with it. So unless you royally eff it up a 4th should extend out high enough to meet the mid/midbass drivers, although (like I mentioned in the 4th thread), you can expect a pretty sharp roll off on the high end.
I'm very happy with my 4th, and I wouldn't hesitate to build another one. That being said, I will be using a rear-wave alignment in my next build.