Basically, any single-edged, folding blade under 4" is OK.
Keep in mind the 4" measurement is arbitrary, so it could apply to the edge length if the cop is having a really bad day.
There are a number of exceptions, the most applicable of which include:
Those with a valid hunting and/or fishing license.
Those who are travelling to or engaging in a martial arts event/class.
Those who are transporting the knife as merchandise intended for sale.
Those who are transporting the knife to or from a repair shop.