Actually the 4-5 fps number is pretty golden. Take a typical bow that's say, 50# @ 28". For sake of this discussion let's say it's 53# @ 29".
So, to accurately measure this you need a shooting machine. Place the bow into the shooting machine and shoot a 10 grain/lb arrow (500 grains) drawn to 28". Let's say it shoots 180 fps.
Now, take the same bow and shoot another 10 grain/lb arrow at 29" (now the arrow must weigh 530 grains since at 29" the bow now pulls 53#). It will almost always shoot 184-185 fps when compared with the 10 grain/lb arrow at 28". So, going up 1" of draw adds (ALL OTHER THINGS REMAINING EQUAL!!) about 4-5 fps.
If we keep the 500 grain arrow that was drawn to 28" (and as a result shot 180 fps) and then pull it to 29" the increment will be more than 4-5 fps. But in this instance the archer has changet TWO things. Not only has the draw length been increased but the grains per pound of draw weight has been decreased. Maybe this way it's possible to get as much as 10 fps difference.
Once again it's important to stress that unless the archer has a perfect release like a super-top-notch archer (think Rod Jenkins) then the only way to measure these differences with certainty is with a shooting machine.