Can you provide some clarification on the situation? It’s a bit unclear, unsure due to typos or something.
So you have a 4yo mini lop and you’d like to introduce a younger rabbit?
Properly bonded rabbits should have a large litterbox for them to share, however, it sounds like your situation is you have an adult rabbit (presumably fixed?) and a new younger rabbit (presumably not fixed). This is problematic, as baby bonds are not the same as a proper bond. The bond between an adult, fixed rabbit and a young, intact rabbit is very unstable and subject to dissolve suddenly and unfortunately sometimes very dangerously due to the young rabbit’s hormones. I recommend housing them separately until both are spayed/neutered, at least 1mo post-op, then you can begin the formal bonding process, as outlined here: https://binkybunny.com/infocategory/bonding/
For information on the dangers of a baby bond, see here: https://wabbitwiki.com/wiki/Bonding_FAQ#My_two_bunnies_used_to_be_happily_bonded,_but_now_they_are_constantly_fighting._What_happened?
The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.