First, how can there be a
Lion King 4 when there hasn't been an "official"
Lion King 3. I know
The Lion King 1½ is titled
The Lion King 3: Hakuna Matata elsewhere in the world, but the official title is still 1½. The official trailer explained why didn't it want to be called TLKIII.
Even though if Disney did want to a sequel to TLKII, it would be titled TLKIII in the United States, but TLK4 elsewhere in the world. Okay, I just wanted to get that out of my system.
Now, to answer your question, there may never be a fourth
Lion King movie because John Lasseter cancelled the production of Disney sequels with
The Little Mermaid: Ariel's Beginning at its last one in the lineup. And playing devil's advocate, if there is one, there's a possible chance it could be about Mufasa and Scar's cubhood.
Second, Kopa doesn't exist in TLK movie universe. The filmmakers on the LK audio commentary named the cub at the end "Fluffy", not Kopa. By the time the production of the second movie was underway, the filmmakers assumingly weren't aware of Kopa's existance, as story director Phil Weinstein claimed he didn't know of him during production as said in this
archived interview. Also, it was confirmed ealier this year that they were going to name Simba's male cub, Chaka, before they wrote him out of the story. So, the only way Kopa could be in the film series is if he is Kiara's younger brother depending if Kiara still wants to be queen.