like a golf cart or something. that would be cool. maybe a 10hp on up engune with a torque converter would work. do you have any welder/fabrication experiences? i can see if there are any sites that may have dimensions and stuff that would help.
That would be a pretty big project. Maybe you could start off with something like a golf cart frame like what biddie said. If I were you I'd really consider the torque converter vs. a regular clutch. You'd need the extra power to start to move that many people.
to save time and money i would build a 4 passenger gocart on slick wheels with a 10hp engine complete with straight pipe, air filter, and torque converter. maybe shocks
10hp??!! its gunna need heaps more grunt!!! i have a 6.5 hp on my tiny little one seater go kart and you think 4 extra ponies is gunna be able to move four people plus the weight of the giant frame. no way in the world.
a friend on mine did something like this with an old motorcycle engine. It was a 2 seater, but he used heavier metal and made an extensive roll cage. He got it running and it went about 45-50 mph. He moved away before he finished it, so i never got to see it complete.