4mm veneer is fine for speaker building
Thanks for the thread, GeorgeO... some of us really need pointers in this area.
I've always used this ply+veneer combo for my speaker projects, because that's what Indian carpenters know when they say "veneer". I have never found a problem. Our western friends get rolls of paper-thin veneer, but why should 4mm be a problem for us? Roll-on veneer is easier to work with than 4mm sheets, because you can use a craft knife instead of a saw. But then in India, most DIYers get a professional carpenter to help with the woodworking (we switch to GID (Get It Done) from DIY whenever needed), so I never faced a problem. And Indian carpenters are experienced in working with the ply+veneer combo -- asking them to switch to the paper-thin type may cause them hardships.
Adding 4mm extra thickness on each side of your enclosure changes the dimensions, but not enough to impact measurements or acoustics. Therefore, you can build using MDF and ply as per plans you have, and then simply add the veneer sheets on all surfaces, and the small dimension change does not impact anything.
It is my belief that an additional wrap of 4mm ply improves the sealing of joints. When we make a box using thick sheets (e.g. my practice of using 25mm MDF everywhere), the joints at right angles may not be as perfect as we want them to be. I generously apply a layer of Araldite on all joining surfaces, but even then, sometimes the carpenter may have missed out a spot, etc. Wrapping the box externally later with a 4mm-thick separate sheet improves the sealing of the box, IMHO, by covering all the inner joints with a new layer... the joints in the veneer are not at the same spots as the joints in the underlying MDF, thereby achieving a kind of double-sealing.
Prices you guys have heard seem to be par for the course -- I used to use Rs.40/sqft sheets when I did the Asawari I seven years ago, but now I am seeing good veneer varying between Rs.80/sqft and Rs.125/sqft. For the Darbari, I have discovered a massive warehouse/ wholesaler of top quality veneer in Vashi, New Bombay. He has a first-floor room with a few dozen raw veneer sheets for display, and they are awesome. These veneers are also far, far better looking than the cheaper teak veneers we used to use most of the time earlier. Therefore, my cost of veneer sheet and labour+parts for PU polish is between 1/3 and 1/2 of my total cost of enclosure construction, but I am happy -- I am getting spectacular appearance as a result.

(My prices are from New Bombay, BTW, not Bangalore.)