Hi and Welcome to SF,
Verify that Netbios is enabled on all computers.
For XP machines: Windows XP Network Troubleshooting
For Vista and 7 machines: http://ecross.mvps.org/howto/netbt.htm
Also, try accessing the shared files by using an IP Address. Click on Start and type \\IPAddress\SharedFolder
What type of Security Software do you have installed in PC4?
You might want to check out this Thread
, it's similar to yours, it might help you guide to some troubleshooting process.