By the time you figure different cab styles, different vehicle option packages, etc. The payload capacity is going to be all over the place for weight. A 4x2 truck has a higher payload capacity than the same style in a 4x4 configuration. Rangers were typical to have 2 or more different suspensions available as well to give different payload options. Toyota has always been very conservative with their numbers because they know that an owner is still going to overload the crap out of their truck, so they stay very conservative in hopes the owner doesn't get too carried away. I've seen some Tacoma's and even the Toyota Trucks with some pretty large travel trailers in tow, that were way over the 4k towing capacity, didn't seem to slow them down any. A lot of these numbers are set in regards to the known stability and capabilities of the vehicle. Many people don't bother to look at the numbers they say I have this truck, and this trailer, I'm gonna tow this trailer, without regards to what the manual says, or safety concerns.
It still doesn't change the fact that they are 1/4 ton rated trucks, be happy it holds 1000lbs, and I bet not a very bright move if you do so either.
You can buy a Ford F-250HD which is technically a 1 ton truck, with 3/4 ton rated springs on it, to keep taxes, registration, and insurance costs down they kept it rated as a 3/4 ton, but has 1 ton axles under it, so in theory a person could in fact swap in 1 ton springs and have an F-350.