I notice on the CPAP.COM site that: "ResMed's HumidAire 4i is engineered to deliver 30% more heated humidification than the H3i. The humidifier is an integrated unit for use with ResMed S8 and S8 II series CPAP machines. The H4i Water Chamber has also increased in capacity up to 390 ml of water." (https://www.cpap.com/productpage/resmed- ... -cpap.html
I find this quite interesting in that the ResMed downloadable User Guides state that the H3i has a capacity of 415ml while the new H4i has a capacity of only 390ml. Hmmmmmm? Gotta' wonder what happened at CPAP.COM
(http://www.resmed.com/us/assets/documen ... er_mul.pdf
(http://www.resmed.com/us/assets/documen ... er_eng.pdf
I was able to take a good look at both the H3i and H4i today at a local equipment supplier. The big difference seems to be the heating surface. The heating surface in the H3i water chamber is a fairly small metal disc situated in the centre of the chamber; this metal disc looks to be fairly thick and sturdy, and rests upon a correspondingly-sized heater plate in the humidifier housing. The H4i has a much larger heating surface, of a fairly thin-looking metal which looks much like that used in the humidifiers of the old "tank" series of Respironics machines. This bottom surface of the water chamber sits on a correspondingly-sized heater plate.
Given that the surface area of the H4i is significantly larger than the H3i, I suspect that the water in the H4i would heat up more quickly and perhaps, stay heated more effectively in the event that the ambient room temperature dropped. In theory, I think that this system would make for more effective humidification. This is just my non-physics brain making a guess --the smart people on the forum will, I'm sure, correct me.
So, while the new H4i should provide more humidity, if the technical specs from ResMed are accurate, the H4i tank does not hold more water than the soon to be discontinued H3i.
The other thing that I noticed is that the housing of the H3i humidifier is clear plastic whereas the H4i has a frosted appearance. The Equipment Supplier person and I both commented that seeing the "max. water level" indicator on the H4i is difficult when compared to the H3i.
Buy a new hat, drink a good wine, treat yourself, and someone you love, to a new bauble, live while you are alive... you never know when the mid-town bus is going to have your name written across its front bumper!