I can think of several reputable ink manufacturers that widely support most Epson models but only a very small number of Canon and HP printers. To name a few, Inktec, Inkrepublic, Cone, Champion, OCP and so on. As a matter of fact, the only manufacturer I know of that makes ink for the newest Canon 12 color models is ImageSpecialists.
With Epson you get more options, both quality and cost-wise.
The canon uses Matte black, Photo black, Pale cyan, Cyan, Pale magenta, Magenta, Yellow, Red, Green, Blue, Grey, Pale grey
The Epson 4900 uses Photo Black, Cyan, Vivid Magenta, Yellow, Vivid Light Magenta, Light Black, Matte Black, Light Light Black, (dont ask thats the stupid name Epson call it), Orange, Green, Light Cyan
To even suggest one over the other is going to be vastly easier to obtain i would guess is silly. Blue if i had to guess would be the hardest for the Canon, Orange and Light, Light Black (i assume or would hope thats more Grey?) for the Epson. I imagine both mean you will have to buy from specific places.
Pointless anyway as i doubt you can easily refill the carts in either machine, and especially not multiple times RELIABLY.
In general, full coverage photographic prints require 1ml of ink per A4 (give or take).
This is just my opinion, but in a commercial environment 4L is a very very small amount of ink.
He is not in a commercial environment hes going to be using the printer heavily for only a few months a year.
The way I have my own equipment set up now for instance, If I were to switch to Canon, I should increase the prices I sell my prints for by a factor of about 2x to compensate for the cost of the printheads.
I do not see how or why, i imagine the waste ink tanks you go through on your Epson cost you more in the long run. You have 2 of them to replace on the Epson 4900.
The way I see it, a set of printheads will cost me as much as the ink that will go through them, and probably more.
You are assuming they will need replacing. Which is unknown.
With my Epsons I have printed several times that, so I am already past the point where anybody can suggest that Canons are the better option because Epsons fail.
Epsons do fail, or rather clog and do it far too easily.
See, I have one 7900 with 40 liters gone through to date, with its printhead costing $1500 or so and still working like the day I bought it. Had I been using a Canon I would have replaced 10 printheads by now, costing me three times the money the Epson head costs...
That maths fails, firstly the 4L if anything (in fact i dunno where you even got that figure) as i pointed out is a GUIDELINE, just as i imagine the GUIDELINE for your Epson is for it to last NOWHERE near as long as it has. You are automatically assuming one is more reliable than the other without side by side testing. You can not say one will exceed manufacturers GUIDELINES and the other will not without side by side consistant testing. I highly doubt your Epson was rated or had a GUIDELINE for 40 litres of ink.
Let's see it from my perspective; to print one 30-spread digital album you'd need about 120ml of ink. So, a Canon printhead would only be good for printing about 30-40 albums. Doesn't look that impressive when thinking it like that.
And, by the way, try repeating your math above; 800 24x26" prints is NOT equivalent to a 19200x20800" print, but to a 19200x26" print, which is about 300 square meters, not square miles, more or less the area of the walls of a typical house
I dunno how you reach the 19200x26" figure, That would be one long skinny print. As for measurements again you are wrongly assuming the Canon would fail.
There are some of us that buy a commercial printer to do commercial work you know. That's what I do and that's what the Canon guys do. It's not unusual to go through tens of liters of ink per year, so the Canons will need their heads replaced once every few months.
Not according to people on this forum which have pounded lower grade Canons for years with thousand upon thousands of prints and never had to touch the printhead.
I am not trying to steer him away from any model. nor do I am in any way affiliated with Epson. I'm stating things as I see them. Epson printers (if they don't malfunction of course) are more economical in the long run than Canons.
I remain unconvinced, theres plenty around the net about the Epson 4900 having issues but not about the Canon i mentioned having to have heads twice in a year.
that is nothing for any half decent printer. 
In fact 16 pages is like its morning warm up while we make coffee.
