I know this is an older thread, however, thought I'd add my 2-cents just the same.
We tow a Palomino Gazelle 230 with a Tacoma 4.0 v6, double-cab, long bed. The "light weight" is about 3700lbs, and as loaded we're at about 4500lbs. The Tacoma v6 is a strong motor, with 3.71 gears, and a Reese SC WD hitch the truck is quite adequate; we are probably close to the realistic capacilty of the truck (not the published capacity), but we don't feel we are overloaded. I would never go any heavier, however; agree with all on that. The Tacoma is quite capable pulling 4500lbs; steeper grades will certainly have you wishing for a larger truck, but mild grades are really not much problem; though you are certainly aware you're pulling that much weight.
By-the-way we get about 20mpg typical, and about 12mpg towing.
I know many of you frown on the smaller trucks, but we've been very pleased with its abilities in the 4000lb area.