RE: 4.0 m2 sail with 400 mast (luff 376)
Windman gives gets it right! And since he's been spending lots of time on the cheap 400-solution he is also more credible than for example me that has been a "mast freak" for years...
In other words, I firmly agree with Sailquick and windman. A 4.0 with a 400 mast is a usable product, but even if the mast and sail are new and top notch, the rig will behave like a 15 year old rig trimmed badly. I frequently sail hear people complaining on 4.0 and 3.5 days that in these winds, the conditions is so on-off, its very gusyt etc. In my experience, this comes from them using the wrong masts. It is simply their rigs that is has a too small wind spectrum where it feels good. Just as Windman writes, with the right mast, a modern 4.0 will be just as active and rangy as a similar 5.0 and 4.0 sailing can be almost as smooth and controlled as 5.0 sailing (except for a typically more difficult water state).
So, the bottom line is that if you can find the money - get the right mast to go with the sail. Maybe you can find a used on cheap. 370 masts generally are closer in character than 400 masts, so finding one that fins should not be that hard. Even if you only get a few days a year with the 4.0, I consider it worth it in the long run (and with a long extension, you might be able to use it on your 4.5 (if you have one) too).
Personally, when I have to pack light for air travel, if I can't "afford" bringing a 370 mast, I don't bring the 4,0 sail either. I rather get by on a perfectly rigged 4.5 (but of course, if it a high wind place I'm going to, I will arrange things so that I can bring 4.0+mast and maybe 3.5+340-mast too).