Here's a link to 3 end mill holders (3/8, 1/2, & 1"), but the draw bar size is 1/2-13.
http://www.use-enco.com/CGI/INLMK32?PARTPG=INSRAR2
I did find a page thru google that had some mills for sale with MT4 spindles but, they're across the pond........ I think.... they had L in front of the price.
http://www.engineerstoolroom.co.uk/turret_mills.htm
If you could find someone here in the states that sell a 7040, see if you can buy an R8 spindle and drawbar from them.
Or maybe find someone overseas that will ship to the USA or maybe someone on the cnczone who will get them for you and ship them here. I don't know how that would work...customs?
Good luck,
Jack
P.S. you might send a e-mail to Little Machine Shop and see if they might have a supplier who makes them. If you don't change the spindle out, you might have this problem everytime you need tooling for your machine.
Walking is highly over-rated