Both :lol:
Behaviour-wise: Dan is bolshie, Blackie is a cute attention seeker, Clo comes running to see if you have food, and Butters is still quite shy.
Looks-wise: Dan & Clo are 'The Big Boys' and Blackie & Butter are 'The Little Boys', cos Dan and Clo are almost twice the size of Blackie & Butter. Clo has much softer fur, and whiter inside his ears. Dan has long legs - we called him a hare when he was a baby :lol: Blackie has a much smaller head than Butter, but from the back it's hard to tell them apart. Also, Butter's ears tend to sit apart, whilst Blackie's ears tend to stay together.
There are lots if little differences if you know what to look for