4. What type of evidence do you think would be most difficult to collect

The type of evidence that is more difficult to collect is evidence known as trace, for example, fibers, hair, or signs of dirt.

Explanation:

For the collection of this type of evidence, specialized equipment such as special vacuum cleaners, tweezers, etc. are used, all with the purpose of carrying out the collection of trace evidence. This type of evidence is characterized by being very difficult to find.

I think it is trace evidence since it is really small and hard to find.