Hey,
the Mini 4 Pro has the C0 certificate. Will this persist if I use third party ND filters? Can I only use DJI ND filters regularly to obtain certification?

The brand of ND filters won't make any difference.
Why would it?

Third-party filters may have a different weight or affect the flight characteristics of the drone. The drone is certified in Germany and European States. The question is whether it will lose it if you mount other parts on the drone.
This may not be relevant in Australia or non-EU countries, but it is in Germany.

Hi there. JeLi64. I hope you are doing well today. You can still use 3rd party ND filters as long as the overall weight does not exceed 250g. Have a great day.

JeLi64 Posted at 10-13 04:29
Third-party filters may have a different weight or affect the flight characteristics of the drone. The drone is certified in Germany and European States. The question is whether it will lose it if you mount other parts on the drone.
This may not be relevant in Australia or non-EU countries, but it is in Germany.

Third-party filters may have a different weight or affect the flight characteristics of the drone.
It would have to be a very, very heavy filter (like 100 grams) to change the flight characteristics.
If the 3rd party filters are a different weight, it would only be by 1 or 2 grams and no-one is weighing your drone or caring if it's a gram over 250g.
If it really concerns you, the solution is to weigh the filters yourself or ask the manufacturer how heavy they are.

Labroides Posted at 10-14 02:21
Third-party filters may have a different weight or affect the flight characteristics of the drone.
It would have to be a very, very heavy filter (like 100 grams) to change the flight characteristics.
If the 3rd party filters are a different weight, it would only be by 1 or 2 grams and no-one is weighing your drone or caring if it's a gram over 250g.

And no-one is weighing your drone or caring if it's a gram over 250g

I would not be so sure in the case of an insurance claim !

Insurance companies like to look for reasons not to pay

Space Dream Posted at 10-14 02:27
And no-one is weighing your drone or caring if it's a gram over 250g

I would not be so sure in the case of an insurance claim !

But a difference of a gram or two won't make any difference to anything and certainly won't cause a serious incident.
If you found yourself in a position where you were being sued for a dangerous incident (which is an extremely rare thing to happen), it would be because you did something dangerous, not because you'd used a filter that was one gram heavier than a DJI filter.

There are some issues that matter.
This is not one of those.

But as I said, if you disagree with me, and want to take this trivial matter seriously, just check the weight of the filters for yourself before endangering everyone around by fitting a filter that pushes your tiny toy drone to 251 grams..

I agree with the others that the miniscule weight difference of a filter will not affect flight characteristics. If you are interested in actual weights I have two very accurate scales (not kitchen scale) that I check with calibration weights and (with my heaviest standard battery) my Mini 4 weighs 248.5 grams with the stock lens cover removed leaving approximately 1.4 grams allowance for an ND filter to safely remain in the lower weight class . The stock lens protector fitting weighs 0.6 grams. Also the batteries have a bit of variation amongst them. I have .24 grams difference between my lightest and heaviest Mini 4 standard batteries.
In Canada the regulations read that the drone must weigh less than 250 grams to remain in the more liberal regulation structure.  
I don't have my mini 4 filters yet but my Neewer Mini 3 filters weigh 1.23gr for the nd's and 1.36gr for the cpl.
So with my M4p on the scale with my heaviest standard battery and my heaviest M3p filter sitting on it it is 249.9g, just barely under the limit. Hopefully the filters I have on order will not be heavier.