Someone mentioned Farid on the first page, and he did make one of his K2 folders with a 4.6" length 5.9mm thick blade in D3. That's pretty close to 1/4", and that K2 was indeed a monster.
For fixed blade EDC-sized knives in 1/4" thicknesses, Busse and Jeremy Horton come to mind, although you could go with other custom makers on the forums as well.
As some people have also mentioned, you should be likely consider more the durability of the pivot if you plan to use it hard. I agree with looking at the ZT30x & ZT200, they are very tough.