I know it's probably something that's been asked a million times but how big do most people feel the difference between 4.0 and 4.2 is? I've been a 5.3 4.5 and 4.0 guy all my sailing life but looking at some new sails and the 4.0 doesn't exist any more. so wondering how big the change will be and how much top end i'll lose on the rare 30knt plus days we get.
Bigger sails can have bigger gaps. When it is really windy and you are getting right down there in size, you need smaller gaps.
So 4 - 4.5 - 5.0 - 5.5 - 6 is not a good quiver - well it is fine, but not ideal.
4 - 4.2 - 4.5 - 4.7 - 5.3 - 5.7 - 6.2 is much better but the cost increases greatly and it probably needs one more mast.
What ranges do a 4.2 but not a 4.0 or smaller now? 
That would be a bit rough, skinny blokes often have a 3.7 and many chick wavesailors need their 3.2's
I don't believe it, if there is a difference it's in the head of the user, 4.2 vs 4 thats twenty square centimeters less than a sheet of A5 paper, Certainly if the two sails are both the same model. I guess one might argue that 20 cm is a whole 5% in area, meh what works for you I guess.
My opinion only, obviously.
Yeah it probably depends on how much money you have to spare plus how consistent the wind is and how skilled a sailor you are.
However on the other hand size increments should be smaller as sails get smaller. So if the wind is a consistent 30 knots where you sail then perhaps its worth considering.
But how often, and in what places, do you get wind so steady that .2m is actually significant?
Given that wind force increases with the square, and gust peaks are normally 1/3 to 1/2 (I've forgotten) as strong as the average, the variation in the power of the wind is so enormous that it must be very, very hard to pin down 0.2.
Add in the other factors - wind sheer, gust/lull stucture, peak v mean v minimum wind strengths, mast compatability, wind temperature/density, sailing style, sailor weight, chop, board, and the vital aspect of settings, and how important is it really?
Just as an example, if it's blowing 4 or 4.2 weather and you're racing, only about 1/3 of the sailors even finish. That is an illustration that there is such a variation in skill that for the average sailor, 0.2 of area seems to be about as important as choosing the "right" colour.
Talking about smaller sails and closer gaps I found this very interesting http://www.iwavesolutions.com/lefebvre/Windsurfing/Articles/quiver.html
Then go to his calculator
www.anyplanet.com/qc/
But what confuses me is if you enter the same sizes but decreasing then the percentages change a bit.
Some people like big percentage spacing to cover more wind range economically and it also makes sail choice easier on the day, others like to have a couple of sails in there most common wind range very close if you damage one then you always have a spare.
I find closer gaps of a small percentage makes the rigging decision harder.
i only go down to a 4.2 but im 110kgs not much use having anything smaller here but i know some guys that use there 3.7 often, theres a big difference when you start going smaller but also depends on what type the sail is, on-shore, freeride, sideshore wave
You lucky, lucky b@stards!
I've had my 4.0 (Gaastra Manic) for a few years now, and I've never used it. Not once. Don't even have a boom short enough, and don't need one. Haven't even used my 5.0 since 2008, come to think of it.
*whimper*
But if u ask me, you can never have too many sails
Thinking of buying a 4.5 but really I am buying it more for the gf and my son to use. Let's see keeping the comparison going will that be a difference of 3 x A3 sheets of paper or 4 x A3 sheets of paper?
I am 67kg. Use a 4.2,4.7,5.2,5.8. All wave sails. Probably only once or twice in ten years that i could not use the 4.2 due to strong winds on east coast NSW. 4.2 is an old Gaastra manic.
I stick to the rule with about 15-20% difference between sails...4.5, 5.3, 6.5 currently, and going for a 3.7 or 3.8 in the near future..
Yeah ... My bad. Note to self, when under the influence of alcohol, don't operate heavy machinery or do simple high school maths, and certainly don't argue about it if challenged. 
To me, the question relates to whether you are going to buy another (370) mast.
4.2 is the smallest size (usually) that fits on a 4m mast.
You can eg. choose a 4.2 sail which handles and depowers a lot with enough downhaul, or a powerful 4.0 onshore wavesail and both would be of about equal power.
Choice is also influenced by which sail you like, how it matches to a particular board, how gusty the wind is and what sea state you would ride it in.
