I'm not absolutely sure, but I don't think they're road-legal in Thailand.
I have seen them on the road, but only out of the city and I've never seen one with a license plate on it.
There are a few falangs around Issan with them, on and off-road, but at least half of the motorbikes in the countryside aren't registered either!
You probably can hire them in many places and I've seen them around too, even on sale in a few malls. Norseman could be right "technically" but we all know about the selective enforcement of traffic laws here, still nothing a couple of hundred baht wouldn't get you out of.
However, from what I here, quad bikes are not the most stable things and can have a realistic chance of flipping over rather too easily on asphalt roads going at higher speeds than what they were designed for doing 'off road' in fields etc.
Yes, there are a couple of places down near Pranburi beach where you can rent them and take them on the beach or road as you please but they don't have license plates on them.
Theres a guy who has one near me with a road plate on it so maybe they can be registered....ask at the cop shop, then the registration place, then your lawyer - bet y get three different answers.
Ive ridden them on and off road in Africa and as long as you remember you are a BIKE not a CAR and act accordingly you should be OK. Try any fast turns and its bye bye
and DO wear a lid eh? It may have 4 wheels but it aint got seatbelts and a roof to roll onto, and if it does roll jump off dont hang on.
"Science flew men to the moon. Religion flew men into buildings."
Thanks for all the replies. So the general consensus is that they are probably not legal. The enquiry was for a friend that is not on this forum, I will pass on the messages.