@The-Hive rod some of this has all ready been discussed mate, it pays to check first as most of the time it’s all ready been discussed on here or if not, have a browse on the internet first as you may find something of value you can post here to help others as well as yourself,
The Mini 3 Pro ND filters are not compatible with the Mini 4 Pro drone .
Yes, you can use the DJI Mini 3 Series Intelligent Flight Battery with the DJI Mini 4 Pro drone . However, please note that when using the DJI Mini 3 Series Intelligent Flight Battery, DJI Mini 4 Pro’s propulsion performance and flight time will be reduced.
Can’t say for certain about the props / holder, internet is a bit conflicting, some say yes some say no but others on here may know better, 