New FIM rules coming .juniors ride 85cc 2stk , 125cc 2stk to keep it affordable. 250 class is 250 class 2stk or 4 strk etcetc.
The 4stk thing started with the Californian clean air act about 20 years ago.
MXA quote" an XR400 winning a superx race"
Then the yz400f got concession to race in the 250 class ,since 500 class was no longer in existance in the US .(and nearly dissappeared in Euro.Joel Smets went door knocking to save it)
The Euro's followed the example in the US and put big bore 4 stk with the 250 2 stk but the Euros are taking the inititive back.
Keep watching .In the meantime I think I'll take my beloved 94 CR250 out to hunt down some 4 bangers at our local.