3 of 4 networked ?

Hello we have 4 computers at home . using cable internet connection via a hub .. we are only able to link 3 of the 4 computers, the fourth computer is able to access all other computers but we are unable to access that one . any suggestions please ??

 

First of all, you might want to check into a switch. hubs are old school. hubs act as simple repeaters, just repeating a signal and sending it out every port, whereas a switch builds an arp table and sends the signal only to the mac address on the port it was intended. also hubs are collision magnets.
but anyway, to answer your question, could be a number of things.
is there a shared folder on the computer you cant access?
is there a firewall option set on the computer you cant access to prohibit incomming connections?
if you have a shared folder and your firewall options are set accordingly, check your shared permissions on the computer you cant access and set the permissions to allow all connections.

this happened to me with my older computer. once i set the permissions on my old comp to allow all connections, it worked fine.