4.5" tip.
if you get the 4" expect to have your neighbors vandalise your car as you will have every window in their house rattling every time you drive down the street.
if you are going to retain the stock turbo and keep to the ~200kw limit that imposes, chances are you wont be able to tell the difference between the two.
that will be an interesting comparison, 98 vs E85 vs sunoco or some such back to back.
and its not like its hard to change tunes DI setup to a toggle switch and it can change fuel and ign maps at the flick of a switch.
running out of meth is only an issue if you don't take at least the basic precautionary measure of installing a level switch in the tank so that you know ahead of time via a warning light that it is getting low.
two fluid systems water/meth mix and petrol, the way the boostcooler setup works is that its boost activated. it only consumes the water/meth mix while you have say more than 6psi of boost or how ever you choose to set it up.