ND filter question

I was looking at buying an ND4 filter, I believe you lose two stops as such, which is fine. However I have seen advertised an ND8. The ad is stating that it is better than an ND2 and an ND4. Is this the case? Why would it be? surely either one has its own uses, could someone enlighten me on this please?
Btw I want to experiment doing long exposure water shots, and whilst reading up on the subject this is where an ND4 was recommended.

Thanx

You're right - they each have their own uses. Basically, the ND8 will let through less light than the ND4 so allowing you to use a longer shutter speed. No one is "better" than the other though.

I'd go with the ND8 if you plan on shooting "slow" water in daylight

So I assume then that if I wanted to take water shots but in a darker situation, say in a dense wooded area I use a lighter nd filter?

and you can stack them too if it is still too light even with ND8