I would rather bathe in two stroke oil than write about modern four stroke motocross bikes. I despise them and everything they stand for. While it's remarkable that Japans' finest have gotten massive power out of a four stroke engine, the result is an engine with a very short life span, and one that costs a fortune to rebuild. The costs associated with the modern machinery slams the door on a kid with asperations of racing, with mom and dad as the sponsors because it's not financially feasible. The result is a sad day for the sport.
Having said this, there was a time when four strokes ruled the motocross track, and that would be up to the late 1960's, when the likes of Husqvarna, CZ, and Maico took over with their two stroke offerings, followed by the Japanese. These old school four strokes were heavy, ill handling, under suspended, loud machines, and infact, they look more like stripped down street bikes, than racing machines.
The bikes of this era were usually based on a proven but modified road engine, which weighed more than a complete modern bike does. It was common for these bikes to have a curb weight of 350 lbs plus. There was no aftermarket back then, so if you were going to race one of these bikes, you had to basically figure out what would make it competitive, design the required parts, make the modifications, and be ready for the weekend race. For the privateer racer of that era, this resulted in being the designer, manufacturer, engineer, research and developement rider, machinist, fabricator, mechanic, race team manager, rider, and driver, all wrapped up in one. The kicker is that their pay day was on race day, and only if they won.
At the same time, most manufacturers had a scrambles team and a comp shop to support the riders, so there were some pretty trick bikes, for their time, raced in this era. While by todays standards they were very heavy, they did put a lot of effort into making the bikes lighter, but there is only so much weight you can cut from a bike of this era. Then purpose built chassis began to become available, as well as complete motorcycles, from the likes of Rickman with their Metisse, Cheney, and CCM.
The pics below are of some of the original four stroke motocross bikes from a bygone era, but it gives you a good idea of the way things were. I must admit the number 10 BSA is my favorite of the below pictured bikes, but all of them could pound a rider to pulp in short order, so lets just say they are beautiful to look at.