The smallest from each manufacture that I know are:

Polaris: Hawkeye 299cc 4x4 auto with IRS
Kawasaki: Prairie 360 (360cc) 4x4 auto with SRA. They use to make a bayou 300 (299cc) 4x4 with semiauto and SRA, but not any more.
Suzuki: Eiger 400 (376cc) 4x4 with auto or semiauto and SRA
Yamaha: Bruin 350 (349cc?) 4x4 auto with SRA They use to make a timberwolf 229cc 4x4 back in 1994, but not any more. I had one and it was good.
Honda: Rancher 350 (329cc) 4x4 semiauto or ESP with SRA. They use to make a 4trax 300 4x4, but not any more.
Artic Cat: They DO make a 250 4x4 semiauto with SRA. I understand it has a 3 range tranny (superlow).

As far as I know, that is all. Tractor supply in Liberty, Texas has an off brand called the FALCON that is 4x4. It is a 260cc 4x4 with auto (H/L range) and SRA. It looks very good and strong for a 260cc class 4x4 and seems pretty large too. I think it is about 550lbs.

 

550lbs!!!! Wow.

 

I could be wrong, but it is somewhere between 500 and 550lbs. It is actually more for an adult, but it has low range and 4x4 to help out. I was told that it actually does well b/c the gearing is very low for the low range and the top speed is around 44mph. But, yes, like you said, WOW......550lbs.