I believe you would need to discard artifacts and defender cards immediately.
Page 16 wrote:
Each Ranger may hold a maximum of 4 Artifacts.... If they gain another, they must choose one Artifact to discard.
For defender cards, you can reference the following thread:
Defender Card Limit and phase 3
And you're correct on special cards.
Page 27 wrote:
A player with 3 Special Cards in their hand may not draw another.