Buy Germ Guardian Filter B which comes with 4 carbon pre filters. These filters are for use with the GermGuardian FLT4825 HEPA filter.
The quality is the same as the one that come with the GermGuardian branded HEPA filter, although the sizing can be a bit different.
You will not have a problem fitting this into the GermGuardian as they are also easily altered simply by cutting them with a scissors if needed.
Germ Guardian Filter B
The material is at least twice as dense as the original filter so it will definitely help to filter out more large particles.
These filters will work for your GermGuardian and are a good value, but be aware they can also be reconfigured for other applications where you might need a carbon filter.
Germ Guardian Filter B work great as another layer to pick up things to help your filters last longer. It’s amazing how much dust and debris it picks up.