No, that is referring to the FIM Motocross rules (
http://www.fim-live.com/fileadmin/alfresco/Codes_et_reglements/Tech_genCMS_en.pdf) (pg 50) for racing a 4$ in the 450 class. You CAN run a 250 4$ in the 450 class but it must be taken out to 290cc. Interestingly, that is not in the 2011 AMA Pro rule book (
http://www.amaproracing.com/assets/AMAPro-MX-2011-Rules.pdf) (pg 20) but it could be an amendment I am unaware of.
For Amateur racing, the 295cc rule does not apply. It just has to be 'over 250cc'. But then again in AMA Amateur racing, there is no difference determined by the number of strokes (except in the Youth classes and in the 125 class - the 125 class being 122 to 125cc 2T only). If your local club is running races with 4$ "handicaps", they are not using the 2011 AMA Amateur rules. They can be found at (
http://www.americanmotorcyclist.com/Libraries/Racing_Documents/2011_AMA_Racing_Rules.sflb.ashx) and classes for MX are on pg 28 and 29.
I was going to comment on it today anyway; The reason you have to run a big bore in the 450 Class is that the 250 4$ is FASTER than the 450 4$ - 2 seconds a lap on most tracks. Look at Hangtown where fastest lap was a 59 and the leader was turning 2:01s on the 450's where the 250s were 58s. (Full Disclosure: Times are not official but taken from Jeff Emig's horrible TV commentary.) It wouldn't do sales any good for the manufacturers to get smoked by a smaller bike.
What I think would be a fun would be to enter an AMA PRO/AM, the classes are much simpler. 122cc to 250cc for the 250 class - 2 or 4 stroke, but that we all know. The interesting part is the Open class where the rules are 122cc to OPEN!
