Single groove, I'll refer to them as chev style, do not touch on the ends, making it such that there is no rotation of the valve possible unless equipped with a rotator.
2 groove style, as used on a Mopar 3/8 stem exhaust valve does not touch, these are used with a rotator so the rotator does the job.
4 groove style as used on a Mopar 3/8 stem valve do touch on the ends, making it such that any movement can exist between the valve and the rest of the components, it can rotate if it wants to.
4 grove version as used on a Ford 11/32 stem valve is not desirable in a performance application because it will wreck the valve grooves of the valve, swap to a single groove for performance app.
Each style has it's own specific need and purpose.
