How do I see them?

I have a total of 4 computers I know are using my connection.
#1 connect wired
#2, 3, 4, always use the wireless

At #1, I can only see #2
At #2, it only show itself

(At #3 & #4, I don't know. They are my elderly neighbor couple I let to use my wireless connection.)

How come I can never see #3 & #4. And, why can't I see all the computers at each computer? These computers are using my connection so they are in my network, right? May be I just don't understand.